Boston's charter school system continued to prove effective at raising test scores and college entrance rates even after doubling the number of students it serves, a study released Monday finds.

The study, authored by three economists and released by the National Bureau of Economic Research, suggests that charters' standardized teaching practices explain the successful scaling up. For charter advocates, however, the study may serve as powerful evidence for the value of expanding the number of kids the schools teach.

The first charter schools opened in Boston in 1994. Since then, they have expanded, today serving tens of thousands of students from the Boston area. On average, Boston charter schools have younger teachers, and are therefore cheaper due to lower salaries and retirement benefits. At the same time, they have been consistently shown to raise SAT scores, AP credits, four-year college enrollment, and other indicators of high school success.

Boston charter schools are also generally known for their pedagogical approach. Termed "no excuses," the approach emphasizes strict discipline, extended schooldays and years, high teacher turnover, and frequent testing with an eye towards using data effectively. The result, the paper's authors claim, has been an increase in test scores among "small groups of applicants, suggesting the potential for transformational effects on urban achievement if these gains can be maintained at larger scales."

The next question, both for Boston's charters and for charters more generally, is one of scalability. Just because a policy intervention works in one particular time or place, with one particular mix of students and teachers, does not mean it will work everywhere. What researchers interested in charters needed was an experiment in expanding the schools, to see if they were able to "scale up" their success.

Just such an experiment came along in 2010, when the state of Massachusetts raised its cap on the share of education funds that could go to charter schools with "proven" track records of success. As a result, the number of charter schools in Boston and the share of sixth graders enrolled in those schools had both doubled by 2015. This doubling was driven largely by the expansion of "proven provider" charters, which created new campuses to serve a broader swath of the population.

To determine if the expansions worked as well as "parent schools" and if they were superior to public schools, the paper's authors took advantage of the random sorting afforded by charters’ lottery admissions. They compared middle school students who were and were not admitted to charters, observing their outcomes at parent campuses, expansion campuses, and in BPS.

"Boston's charter sector," they write, "remained effective while doubling in size." At both expansion and non-expansion schools, students outperformed their BPS peers by a significant amount on math scores, and by a notable but not significant amount on English scores. What is more, the new campuses "generate test score gains similar to those of their parent campuses despite a doubling of charter market share in middle school," the authors write.

This finding is particularly impressive because of the disparities between new and old charters, and between new charters and BPS. The average teacher at an expansion charter had just 1.4 years of teaching experience; at a parent charter, 2.9; and at within the Boston Public System, 11.5. What is more, because the 2010 law instructed charter schools to actively recruit more students without English language proficiency or with special education status, the new campuses were teaching a more challenging mix of kids than the original ones.

This, the authors conclude, indicates that the new success is not a function of positive selection of the best kids into the schools. They also rule out the possibility that comparison to worse "fallback schools" attended by those who are not admitted explains their results. What, then, explains the scalability of Boston's charters?

"Though changes in demographic composition contributed modestly to the positive impacts of new charters, neither changes in the student body nor the quality of applicants' fallback traditional public schools explain the pattern of results," the authors write. "Instead, it appears that proven providers successfully transmitted hiring and pedagogical practices to new campuses."

The charter system, in other words, standardizes pedagogical practice to create a more uniform experience for students. The relative youth and high turnover rate of the charter school teacher pool leads to a need for standardized training and communication of best practices between older teachers and younger—what the paper calls "charters' centralized management of teachers and standardized instructional practices."

This approach gives young teachers less discretion, but also ensures that they are using methods that appear to work. The result is that most charter teachers use the same pedagogical approach—and that pedagogical approach appears to be readily translatable from old school to new.

The evidence that charter schools can "scale up," doubling in size without taking a hit to their effectiveness, will be welcomed by the oft-criticized education-reform movement, which sees charters as a crucial tool for closing the achievement gap.

The papers authors offer a second insight, writing, "in the charter school context, replicating existing charters may be a better option than allowing new providers to enter the sector." In other words: if cities want to proceed with expanding their charter system, they should look to those schools that already have a proven track record of success.