Sunday Music: Mercy, Mercy Mercy

Oh, my:



Kills me we don't have video of Cannonball playing this. The slideshow on this video is good, though.

I played this today at a Jazz Vespers series I'm doing in Morristown. Stop by on Sundays this month if you feel the spirit.

More About Attrition Rates in Boston Charter Schools

The debate over Massachusetts's Question 2 -- a referendum on lifting the cap on the number of charter schools in the commonwealth -- rages on. When I last weighed in, I pointed out that the "successes" of Boston's charter sector could not fairly be compared to the "failures" of the public schools because the two sectors were educating fundamentally different students.

One indicator of this is the cohort attrition rate: the shrinkage in the size of a cohort that occurs because students leave a school, but are not replaced with new students entering.

Here's a school that "backfills"; in other words, as students leave because their families move, or they drop out, or they transfer, or whatever, they are replaced by new students entering the system.

This school doesn't backfill. What happens to the size of their cohorts (another way of saying "Class of x")? They shrink -- and that's what appears to be happening in Boston's charter schools.

A quick note: there are two kinds of charters in Massachusetts. "Commonwealth" charters are independent of their host districts, while "Horace Mann" charters are managed within the district. The  debate over Question 2 is really focused on Commonwealth charters. Kennedy Academy in the chart above is a Horace Mann charter; the others, in red, are Commonwealth charters. In Boston, Commonwealth charter high schools have far greater cohort attrition than the public district schools or the Horace Mann charters.

Furthermore, this attrition is part of a pattern across time:

Year after year, the cohorts enrolled in Boston's charter high schools shrink much more than the cohorts in the public district schools. Why? The simplest explanation is that the charters are not backfilling at the same rate as the district. This is a clear indication that the students in Boston's charters are not the same as the students in the district: their families are less mobile, and they are more likely to be a better "fit" with the philosophies of the charters than the kids who left.

This is one reason I contend you can't make a comparison between the achievement of the two groups of students, even if you use a randomized controlled trial design: the kids who attrit from the charters are likely quite different from the kids who remain.*

Now, a reader who comments over at Diane Ravitch's blog pointed out that there might be another explanation for all this: maybe the charters are retaining students for an extra year to get them "caught up." If that's the case -- and, certainly, some Boston Commonwealth charters do advertise the fact they will retain students if they think it's warranted -- then the cohorts would appear to be shrinking over time, because there would be more freshman than upperclassmen, even if none of the students left.

It's an interesting theory; however, I think there's plenty of evidence to show retention does not explain all, or even most, of the cohort shrinkage found in Boston's charter schools. Let's start with this:

This is the "intake" rate for Boston's charters and the Boston Public Schools (BPS) in 2016. According to the MA Department of Elementary and Secondary Education: "The Intake Rate measures the number of students that enroll in the state, a district, or school after the beginning of the school year..." In other words, who is taking more of the students who are coming into schools mid-year?

The answer is clear: BPS and the Horace Mann charters have much greater intake rates than the Boston Commonwealth charters. Granted, this doesn't tell us much about the intake that occurs between school years; however, it does again provide some evidence that the students in BPS and the charters are not the same.

Next, let's look at how the cohorts shrink in BPS:

As each cohort progresses from freshman to senior year, it shrinks. Some of that is due to dropping out; some is due to student mobility; some is due to transfers out of the system and into other public or private systems; some may even be due to retention.

There's a good case to be made that BPS has made substantial improvements in its ability to retain students through high school:

The senior class of 2005 was only 65% of its size in its freshman year; that's improved to 85%. So, now that we have some context, let's look at the same data for some Boston charter schools. We'll start with the largest of the high school charters, City On a Hill:

If COAH was retaining students in their freshman year, we would certainly expect a drop in students from freshman to sophomore years. But then we'd expect to see the number of students level off between Grade 10 and Grade 12. Except that's not the case; COAH's cohorts continue to shrink in their upperclass years.

Let me show it in numbers; here are the cohorts and their enrollments for grades 9 to 12:

Yes, the cohorts shrink significantly between Grade 9 and Grade 10, but then they shrink again between Grade 10 and Grade 11. And most also shrink between Grade 11 and Grade 12.

I find it very unlikely that retention explains this. And even if it did: is that really a good thing? How many years does it take to graduate from COAH? Five? Six? More?

Here's the cohort shrinkage for Match Charter:

Again, in most cohorts, the attrition that starts from Grade 9 to Grade 10 continues in the upper grades. I've posted some more examples below, but let's step back a bit and assess all this before we get to them.

Yes, the cohorts shrink for BPS -- but they shrink much more for the high school charters. And they shrink through the four years of high school, suggesting that retention isn't the sole explanation; if it were, students are being retained more than one year, which is hardly ideal.

I would argue this data is yet another piece of evidence that throws into doubt the claim that Boston's charter high schools can easily scale up their "successes." Even if they are retaining students, these charters schools are seeing their cohorts shrink substantially. Clearly, after gaining admission, many of their students are "voting with their feet" in years other than their freshman year -- and they are not being replaced by the students who are, allegedly, on waiting lists.

There's one other thing worth noting here -- something you'll miss if you don't pay attention to the vertical scales of my graphs:

As I pointed out before, the Commonwealth charter schools are a tiny fraction of the total Boston high school population. What happens if the cap is lifted and they instead enroll 25 percent of Boston's students? What about 50 percent? 

Let's suppose we ignore the evidence above and concede a large part of the cohort shrinkage in charters is due to retention. Will the city be able to afford to have retention rates that high for so many students? In other words: what happens to the schools budget if even more students take five or six or more years to get through high school? 

In a way, it doesn't really matter if the high schools get their modest performance increases through attrition or retention: neither is an especially innovative way to boost student achievement, and neither requires charter school expansion. If Boston wants to invest in drawing out the high school careers of its students, why not do that within the framework of the existing schools? Especially since we know redundant school systems can have adverse effects on public school finances?

I'll have more to say about Question 2 in a bit. For now, here are some more cohort enrollment patterns for Boston charter high schools:

Again: it would be very hard to explain all of these solely through retention. But even if we could: that still isn't an argument for raising the charter school cap.

More to come -- stand by...  

* Nerds, we'll talk about instrumental variable estimation and attrition soon, I promise.

Charter Schools and the Talented Tenth

Every once in a while -- if you listen carefully -- you can hear charter school supporters give away their game...

Take, for example, Shavar Jeffries, president of Democrats for Education Reform, one of the most influential pro-charter lobbying outfits in the country. Jeffries was put out by yesterday's resolution from the NAACP calling for a moratorium on charter school expansion. It's clear Jeffries sees the NAACP's position is a betrayal of its core mission -- but notice who he evokes to make his case:

OCTOBER 15TH, 2016

New York, NY — Following today’s vote by the NAACP to approve a proposed moratorium on all charter schools, DFER President Shavar Jeffries released the following statement:

“W.E.B. DuBois is rolling in his grave.  The NAACP, a proud organization with a historic legacy of expanding opportunity for communities of color, now itself stands in the schoolhouse door, seeking to deny life-changing educational opportunities to millions of children whose parents and families desperately seek alternatives to schools that have failed them for too long.  Public charters schools throughout the country are creating new pathways to college and career that were previously unavailable.  The idea that the NAACP would support a blanket moratorium that would apply across-the-board to all charters, including schools like Urban Prep that send 100% of its graduates to college, is a tragic contradiction of what the NAACP has traditionally stood for. The NAACP faces a choice: cling to policies of the past that have failed Black children for decades, or embrace the future and the innovative practices that will create hope and opportunity in places where neither is present.  We will continue to stand with Black parents and families throughout the country, as well as leaders of color, including President Obama, in supporting any policy in any form that creates equity and opportunity for children of color. The NAACP has chosen a different path, and history will record its choice.” [emphasis mine]

Let's talk a little about DuBois and education:

I've actually been doing a bit of reading on the debates surrounding the education of African Americans that took place in the wake of the Reconstruction Era. James Anderson wrote a brilliant book, The Education of Blacks in the South, 1860-1935, that digs deep into this history.

The standard trope about this era is that there was a debate between DuBois and Booker T. Washington that pitted DuBois' desire for blacks to pursue a classical liberal education against Washington's call for vocational training.

Anderson shows this is far too simplistic: in fact, very few in the black intelligentsia sided with Washington. But as DuBois makes quite clear in his famous essay from 1903, the elite education he favored was just that: elite. DuBois clearly and unapologetically was calling for a particular sort of education that would develop the best and the brightest -- the "talented tenth." (all emphases mine)

The Negro race, like all races, is going to be saved by its exceptional men. The problem of education, then, among Negroes must first of all deal with the Talented Tenth; it is the problem of developing the Best of this race that they may guide the Mass away from the contamination and death of the Worst, in their own and other races. Now the training of men is a difficult and intricate task. Its technique is a matter for educational experts, but its object is for the vision of seers. If we make money the object of man-training, we shall develop money-makers but not necessarily men; if we make technical skill the object of education, we may possess artisans but not, in nature, men. Men we shall have only as we make manhood the object of the work of the schools–intelligence, broad sympathy, knowledge of the world that was and is, and of the relation of men to it–this is the curriculum of that Higher Education which must underlie true life. On this foundation we may build bread winning, skill of hand and quickness of brain, with never a fear lest the child and man mistake the means of living for the object of life. 

If this be true–and who can deny it–three tasks lay before me; first to show from the past that the Talented Tenth as they have risen among American Negroes have been worthy of leadership; secondly to show how these men may be educated and developed; and thirdly to show their relation to the Negro problem.

 DuBois continues:

Can the masses of the Negro people be in any possible way more quickly raised than by the effort and example of this aristocracy of talent and character? Was there ever a nation on God’s fair earth civilized from the bottom upward? Never; it is, ever was and ever will be from the top downward that culture filters. The Talented Tenth rises and pulls all that are worth the saving up to their vantage ground. This is the history of human progress; and the two historic mistakes which have hindered that progress were the thinking first that no more could ever rise save the few already risen; or second, that it would better the uprisen to pull the risen down. 

All men cannot go to college but some men must; every isolated group or nation must have its yeast, must have for the talented few centers of training where men are not so mystified and befuddled by the hard and necessary toil of earning a living, as to have no aims higher than their bellies, and no God greater than Gold.

Further:

I am an earnest advocate of manual training and trade teaching for black boys, and for white boys, too. I believe that next to the founding of Negro colleges the most valuable addition to Negro education since the war, has been industrial training for black boys. Nevertheless, I insist that the object of all true education is not to make men carpenters, it is to make carpenters men; there are two means of making the carpenter a man, each equally important: the first is to give the group and community in which he works, liberally trained teachers and leaders to teach him and his family what life means; the second is to give him sufficient intelligence and technical skill to make him an efficient workman; the first object demands the Negro college and college-bred men–not a quantity of such colleges, but a few of excellent quality; not too many college-bred men, but enough to leaven the lump, to inspire the masses, to raise the Talented Tenth to leadership; the second object demands a good system of common schools, well-taught, conveniently located and properly equipped.

Now, it is certainly true that DuBois, no doubt feeling he had to respond to his critics, adjusted his ideas about the "talented tenth" later in life. In 1948, he specifically addressed how his thinking about the "talented tenth" had evolved:

Turn now to that complex of social problems, which surrounds and conditions our life, and which we call more or less vaguely, The Negro Problem. It is clear that in 1900, American Negroes were an interior caste, were frequently lynched and mobbed, widely disfranchised, and usually segregated in the main areas of life. As student and worker at that time, I looked upon them and saw salvation through intelligent leadership; as I said, through a "Talented Tenth." And for this intelligence, I argued, we needed college-trained men. Therefore, I stressed college and higher training. For these men with their college training, there would be needed thorough understanding of the mass of Negroes and their problems; and, therefore, I emphasized scientific study. Willingness to work and make personal sacrifice for solving these problems was of course, the first prerequisite and Sine Qua Non. I did not stress this, I assumed it.

SACRIFICE

I assumed that with knowledge, sacrifice would automatically follow. In my youth and idealism, I did not realize that selfishness is even more natural than sacrifice. I made the assumption of its wide availability because of the spirit of sacrifice learned in my mission school training.

In my reading, DuBois didn't call for the removal of an African American aristocracy; he just wanted a better one:

This screened young membership must be far greater in number than it is now. Baltimore for instance has more than 166,000 Negroes and only 23 in its Boule, representing less than 100 persons. Surely there must be at least 23 other persons in Baltimore worthy of fellowship. It is inconceivable that we should even for a moment dream that with a membership of 440 we have scratched even the tip of the top of the surface of a group representative of potential Negro leadership in America. Nothing but congenital laziness should keep us from a membership of 3,000 by the next biennium without any lowering of quality; and a membership of 30,000 by 1960. This would be an actual numerical one hundredth of our race: a body large enough really to represent all. Yet small enough to insure exceptional quality; if screened for intelligent and disinterested planning.

So DuBois, even in his later years, believed in the education of a "screened" group of "exceptional" black students whose mission would be to become leaders: "The Negro race, like all other races, is going to be saved by its exceptional men."

Even if I was a DuBois scholar, I wouldn't attempt to guess what he would think about today's charter school movement. Certainly, he would have had a problem with veteran black teachers being replaced by novice white teachers the urban areas that have seen the rise of charter schools. Given his embrace of Marx, it's hard to believe he'd be fine with private companies swooping in and converting public institutions like schools into private concerns.

But would he have actually embraced the idea of charter schools serving the "talented tenth"? He certainly had no problem cultivating a small percentage of talented students of color, educating them with the goal of turning them into his race's leaders. Would he have wanted motivated families with talented children to be sequestered in their own schools? It certainly seems possible.

In evoking DuBois, Jeffries is, in effect, asking this very question: should there be a separate system for what Michael Petrilli, as big a charter cheerleader as you will find, calls the "strivers":

Frustrated that the traditional public schools aren’t willing to prioritize their children’s needs, many low-income strivers have turned to high-quality charter schools instead. But now those are under attack, too. Both the PBS Newshour and the New York Times have recently presented highly critical coverage of Success Academies, charter schools in New York City that have shown excellent results in improving student performance. The reports focused on the academies' suspending students aggressively and pushing out chronic disrupters. There were similar controversies over the relatively high rates of suspensions and expulsions at charters in Chicago and Washington in recent years.  

The casual observer might wonder: What’s wrong with that approach? Why not ensure that schools are safe places to be? If the Success Academies and schools like them didn’t exist, many of those hard-working, high-achieving students would be in chaotic, low-performing public schools. Don’t their needs count? [emphasis mine]

This is a "talented tenth" argument if I ever heard one (although maybe Petrilli would argue about the actual percentage). I've given Petrilli credit before for daring to go where most in the charter industry won't: the biggest selling point that urban charter schools make to parents isn't "innovation" and "freedom" -- it's peer effects.

We see this in the high suspension rates of Uncommon's charter schools. We see it in the classroom practices and discipline policies of Success Academy. We see it in the "no excuses" philosophy of KIPP (to their credit, some of the KIPP schools seem to be rethinking this). Despite all their protests to the contrary, these "successful" urban charter schools are attempts to foster elitism. They exist not to educate everyone; they exist to educate the "talented tenth."

Now, there's a legitimate critique to be leveled against white guys like me living in the suburbs who express our reservations about all this: who am I to tell people of color how to run their schools? Who am I to tell a family that wants to do right by their children that they shouldn't have the opportunity to send their child to a school with positive peer effects? Why should my kids get the advantages of being surrounded by lots of college-bound peers while other kids aren't?

It's a fair point, and it's why I never fault a parent for sending their child to a charter school; everyone has to do what's best for their kids.

But if charter cheerleaders like Jeffries are going to promote "choice" by evoking DuBois, I think it's more than fair to start asking a few tough questions:

- If charters are serving the "strivers," why pretend they are doing the same job as the public schools? Why pretend that they've found some sort of secret sauce that leads to improved outcomes when the real secret may be unmeasured -- but real -- differences between students?

- If the primary goal of urban education going forward is to serve the "talented tenth," what happens to the other 90 percent? What sort of schooling to they deserve? Why don't they get the resource advantages of "successful" charters? Why should their public schools suffer just to make sure charters get a bigger slice of the pie?

- Why do the charters need to be privately run? If their "success" is due to different student populations (and, to some extent, resource advantages), why allow them to operate under a set of rules that actually incentivize behaviors that aren't always in the public interest?

- Why is it OK to segregate the strivers from everyone else in the urban schools, but not in the suburbs? As I've said before, there are plenty of kids in the leafy 'burbs who have emotional, cognitive, and other learning impairments. But unless they are extreme cases, suburban schools go out of their way to integrate these children in with their peers. Why should this be the case in some schools but not others?

- If charters are serving the "strivers," why are so many of them engaging in pedagogical and disciplinary practices that do little to foster student agency and social justice? Why do so many "successful" charters have high attrition rates? And high suspension rates? And a curriculum that is narrowly focused on test scores and disturbingly teacher-centered? Is this really the best way to develop leaders?

- Finally: yes there are some "successful" charters. But they are not the norm.

If you look at the total charter sector, the results are underwhelming, and profit-taking runs rampant. How does it serve anyone -- in or out of the "talented tenth" -- to enroll students in the many charter schools that put profits before the needs of students?

I'm glad Shavar Jeffries decided to quote W.E.B. DuBois -- his words are well worth considering as we continue to ponder the growth of the charter school sector.

ADDING: As I've said before: the fundamental problem with the "reform" argument is that the "reformers" always conflate social equity with social mobility. They are not the same. 

ADDING MORE: Julian Vasquez Heilig reminds us that DuBois was not a big fan of the privatization of governmental roles:

• The position of the NAACP and Black Lives Matter on privatization is consistent with the views of past civil rights leaders. NAACP co-founder W.E.B. Du Bois, in his essay Negroes and the Crisis of Capitalism in the U.S., extolled the virtues of collaborative social and government action. He railed against the role of businesses and capitalistic control that “usurp government” and made the “throttling of democracy and distortion of education and failure of justice widespread.” Malcolm X characterized market-based public policy  as “vulturistic” and “bloodsucking.” He advocated for collaborative social systems to solve problems. Martin Luther King Jr. argued that we often have socialism in public policy for the rich and rugged free market capitalism for the poor. King and Malcolm X would have recognized the current patterns we see of charters located primarily in urban and poor areas rather than wealthy suburban enclaves. White academics pressing for market-based school choice in the name of “civil rights” ignore this history of African American civil rights leaders advocating for collaborative systems of social support and distrusting “free market” policies.

Charter School Attrition in MA: A Reader Responds

A week ago, I wrote a post about attrition rates in Massachusetts charter schools. The issue is important as the state will vote soon on Question #2, which would lift the cap on charter school proliferation. If charters lose students at inordinately high rates, there's a good chance allowing them to expand won't improve student outcomes, and could instead cause harm to public schools.

Diane Ravitch often comments on my stuff and provides a link, and did so with this post. Her readers are more inclined to leave comments than mine, and a discussion about attrition in MA's charters ensued. Stephen B. Ronan, a frequent commenter at Diane's site, left a particularly long one; he also posted here.

With his permission, I am reposting his comment below. I'm doing so because he raises many of the same points I've read in the larger debate over Question #2, and I thought it would be useful to have them all in one place so I can answer them in subsequent posts.

Leave your comments below; mine will follow over the next few weeks, with the exception of the following:

- I didn't provide a link to the MCPSA document in my original post. That was an oversight; regular readers know I post primary source links regularly. I've added the link to the original post.

- Phoenix Charter Academy is, indeed, in Chelsea and not Boston. Again, I corrected that in the original post, and apologize for the error. That said, excluding Phoenix does not substantially alter my original analysis.

* * *

It looks to me, JJ, as if you may have some misunderstandings that may be worth correcting.

Without providing a link, you quote the Massachusetts Charter Public School Association.
I think this is the full document for those who may be interested:
http://www.masscharterschools.org/media/news/just-facts-public-charter-schools-massachusetts

The segment you quote from states:

"The attrition rate at Boston charters (9.3%) is significantly lower than in BPS (14.2%).
In Gateway Cities, charter attrition rates (6.2%) are lower than Gateway districts (11.4%).
"From 2012-2014, an average of just 82 students left charters and returned to Boston Public Schools, according to BPS numbers – one-tenth of one percent of BPS total enrollment of 57,000."

And you responds to that:
"Yeah, uh... no. Not really."

You claim, properly, that our department of education's "attrition" rate is inadequate to provide a full perspective insofar as it addresses students leaving during the summer. What you may not understand is that the sentence that starts "From 2012-2014" is supplementing the summer attrition figures by citing the number of students who leave charter schools to return to BPS schools not during the summer but rather during the school year. I think the Association's point there is that there's not much such movement during the year relative to summer attrition which is when most of the mobility between sectors occurs.

Our state department of education makes very easily available both summer attrition rates and also measures that show the rates at which students leave during the year. For example, to view the summer attrition and the school year stability rates of one superb charter school and one superb traditional public school, see;

MATCH attrition: http://profiles.doe.mass.edu/attrition/default.aspx?orgcode=04690000&fycode=2015&orgtypecode=5&
MATCH stability: http://profiles.doe.mass.edu/mobility/default.aspx?orgcode=04690000&fycode=2015&orgtypecode=5&
Nathan Hale attrition: http://profiles.doe.mass.edu/attrition/default.aspx?orgcode=00350243&fycode=2015&orgtypecode=6&
Nathan Hale stability: http://profiles.doe.mass.edu/mobility/default.aspx?orgcode=00350243&fycode=2015&orgtypecode=6&

You write: "If charter schools shed kids year after year -- especially if those kids are low-performing -- then their vaunted performance advantages are in question, particularly when compared to public district schools that aren't losing students."

Yes, but in fact the evidence demonstrates that charter schools in this area shed considerably fewer kids than public district schools.

I guess you recognize that may well be the case so move swiftly on to change the subject:
"What we really need to consider is whether the students moving out of charter schools are being replaced at rates equal to the replacement rates for students moving out of public district schools."

Your conclusion there is correct: there's far less backfill in the charter sector.

You write: "Every independent charter school in Boston had a higher cohort attrition rate in 2014 than BPS as a whole."

Is this a specifically New Jersey use of the phrase "cohort attrition rate"? I find it difficult to locate a definition in the academic research literature that combines what is normally considered to be attrition (loss of students) with backfill (gain of students) and calls the merged product "attrition". Though I do see it used that way by both you and Bruce Baker and in New Jersey.

In respect to what may constitutes a "cohort" in respect to individual schools in Boston, it appears that you blend members of multiple entering classes, and of multiple cities. For example, you write:

"In the case of City on a Hill and Phoenix, their 2011 freshman class shrank by more than half by the time they were seniors."

Phoenix doesn't have a school located in Boston. Perhaps you are thinking of the Phoenix charter school not too far away in the city of Chelsea. But given the nature of the school's specialization it makes it doubly hard to justify comparing it with a traditional public school in Boston:

"The school she founded, Phoenix Charter Academy, was conceived as an alternative school for students who had given up on traditional high school, or who felt that their schools had given up on them. As it celebrates its 10-year anniversary this week, it remains a haven for students who thought they might never finish high school."

https://www.bostonglobe.com/metro/2016/03/31/chelsea-charter-program-that-reached-struggling-kids-reaches-milestone/KYZKbHYHdsqCVD6XM8xxeI/story.html

As for City on a Hill, you omit the implications of this, from its annual report: "There is no social promotion at City on a Hill. 100% of students promoted to the next level in each subject demonstrated mastery of the school’s common-core aligned college prep curriculum by earning 70% or above on written and oral proficiencies."

http://www.cityonahill.org/wp-content/uploads/2015/08/City-on-a-Hill-2014-15-Annual-Report-Final.pdf

Presumably as a consequence of that policy City on a Hill has extremely front-loaded grade level retention. The 9th grade class enrollments you examine merge students who are newly starting out and others who are repeating the grade. That distorts any attempt to compare 9th grade and 12th grade enrollment figures to measure attrition.

Massachusetts grade-level retention reports can be found here:

http://www.doe.mass.edu/infoservices/reports/retention/

Looking at Appendix B for 2015-2016, one finds for City on a hill the following grade-level retention rates:
9th 10th 11th 12th
27.8 3.3 3.2 7.4

In the past, in the several messages here, you and I discussed whether such a front-loaded grade-level retention pattern skews the results you and your NJ colleagues try to portray:

https://dianeravitch.net/2016/07/14/jersey-jazzman-takes-charter-cheerleaders-to-the-woodshed-for-destroying-public-schools/#comment-2582329

I continue to believe it does.

The one clearly valid point you make, not a novel one, is that there's less backfill at charter schools here in Massachusets charter schools than at traditional public schools. I'd be interested to see you debate with Michael Petrilli on whether or not that's helpful:

https://edexcellence.net/articles/backfilling-charter-seats-a-backhanded-way-to-kill-school-autonomy

You write: "Is it really worth expanding charters and risking further injury to BPS when the charter sector appears, at least at the high school level, to rely so heavily on cohort attrition?"

Further injury? Could you explain what you meant by that? This has what has been transpiring at BPS in parallel with the growth of charter schools:

http://www.bostonpublicschools.org/domain/238

* * *

ADDING: Ronan also left this comment at Ravitch's site:

To save us some back and forth, I would suggest that in your response you also consider the material referenced here in respect to attempts to ascribe high test scores at charter schools to attrition:
https://dianeravitch.net/2016/09/20/george-joseph-the-racism-of-charter-school-discipline-policies/#comment-2597509
And both of these documents in respect to hypothetical financial injury to BPS schools:
http://bmrb.org/wp-content/uploads/2016/04/SR16-2Charter.pdf
http://manhattan-institute.org/html/massachusetts-charter-school-cap-pro-con-9278.html
The latter, which is brand new, states: “In 2016, charter enrollment had the effect of increasing per-pupil spending in district schools by approximately $85 million statewide.”
It has both strengths and weaknesses and you may find it, more generally, worth addressing together with the Brookings study.
You may find this useful: http://www.doe.mass.edu/research/reports/2016/02CharterReport.pdf
Finally, if you have time and space, you may wish to consider how passage or failure of our Q2 may impact likelihood of passage by the voters in 2018 of our millionaire’s tax proposal: http://www.bostonmagazine.com/news/blog/2016/05/17/what-is-millionaire-tax-massachusetts/
Best wishes.