Backfilling charter school seats didn’t hurt test scores, new Boston study finds

Mary McLeod Bethune Day Academy is one of 123 schools in the Washington, D.C., area under the oversight of the D.C. Public Charter School Board. Founded in 2003, the PreK3-8 academy is an IB World School that offers world language, art and music in addition to its core academic curriculum.

Editor’s note: This analysis from Marcus A. Winters, an associate professor and chair of the department of educational leadership and policy studies at Boston University’s Wheelock College of Education and Human Development, appears in the Summer 2022 issue of Education Next.

Among the regulatory freedoms available to most charter schools is the ability to restrict the entry of new students. Oversubscribed charter schools must offer seats randomly, and thus they can’t choose among applicants.

But most charter schools can choose whether to “backfill” vacancies created by students who leave. Though there is no reliable estimate for the number of vacant seats in charter schools, there is ample reason to believe that decisions not to backfill substantially reduce charter school enrollments in several localities. For example, of the 53 New York City charter schools analyzed in a 2015 report by the Independent Budget Office, 20 replaced fewer than 70% of students who left.

Despite relying on per-pupil funding, many charter schools choose not to backfill because they are concerned that the disruptions caused by incorporating new students would reduce their effectiveness for the students already enrolled in the school. Many think that incorporating new students would be especially disruptive within charter schools that apply models that emphasize developing a consistent culture and norms over time.

Fordham Institute President Michael Petrilli summarized this argument well in a 2015 commentary:

“ …there are strong instructional arguments for not backfilling. Great schools spend a lot of time building strong cultures—the almost-invisible expectations, norms, and habits that come to permeate the environment, such as the notion that it’s cool to be smart and it’s not OK to disrupt learning. Culture-building is a whole lot harder to do if a school is inducting a new group of students every year in every grade …. This isn’t just a technical challenge; there’s a moral question too. Backfilling is surely good for the student who gets to claim an empty seat. But what if it’s bad for their new peers? What if the disruption to the many outweighs the benefits to the few?”

Though the argument has intuitive appeal, whether backfilling enrollment harms incumbent charter school students has remained an open question. In a new paper for the Wheelock Educational Policy Center, my co-author Cheonghum Park and I fill this gap in our understanding of charter school impacts by measuring the causal effect of required backfill on the achievement of incumbent students within Boston’s charter middle schools.

Our study leverages a 2010 change in the law governing charter schools in Massachusetts. In addition to other reforms, the new law required charter schools to begin backfilling vacancies that occur within the bottom half of grades offered within the school.

In practice, the change in the law didn’t impact enrollments for charter schools operating outside of Boston because they were already backfilling vacancies at very high rates. However, Boston charter schools backfilled very few vacancies prior to the change in the law, and we show that introduction of the requirement had a substantial impact on their enrollment patterns.

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