What we know about public charter schools can be summed-up with two stylized facts. One is that on average charter schools are about average. The second is that the best charter schools are really amazing, achieving extraordinary results with students from challenging demographic subgroups. One natural question to ask about that is what, exactly, is it that the best charter schools are doing that makes them work well. From a journalistic perspective, people often like to give qualitative accounts of the best-performing schools and then just point to unusual things they do. But Will Dobbie and Roland G. Fryer try to get under the hood in a more statistical sense in "Getting Beneath the Veil of Effective Schools: Evidence From New York City" by testing for statistically significant correlations between different attributes and charter performance:
In this paper, we collect unparalleled data on the inner-workings of 35 charter schools and correlate these data with credible estimates of each school's effectiveness. We find that traditionally collected input measures—class size, per pupil expenditure, the fraction of teachers with no certification, and the fraction of teachers with an advanced degree—are not correlated with school effectiveness. In stark contrast, we show that an index of five policies suggested by over forty years of qualitative research—frequent teacher feedback, the use of data to guide instruction, high-dosage tutoring, increased instructional time, and high expectations—explains approximately 50 percent of the variation in school effectiveness. Our results are robust to controls for three alternative theories of schooling: a model emphasizing the provision of wrap-around services, a model focused on teacher selection and retention, and the "No Excuses'' model of education. We conclude by showing that our index provides similar results in a separate sample of charter schools.
This follows on Dobbie & Fryer's specific research on the Harlem Children's Zone where they similarly found that the wraparound social services are not the key element to HCZ's success. This is hardly the last word on the subject, since it's not the biggest sample you can imagine, but it seems important. In particular, the trade-off between smaller class sizes and extending learning (a trade-off because both require larger expenditures on teacher compensation) isn't one of our hot-button "education wars" issues but the research on this seems really clear to me and keeps popping up in different kinds of research.