Do small schools improve performance in large, urban districts? Causal evidence from New York City

We evaluate the effectiveness of small high school reform in the country's largest school district, New York City. Using a rich administrative dataset for multiple cohorts of students and distance between student residence and school to instrument for endogenous school selection, we find substantial heterogeneity in school effects: newly created small schools have positive effects on graduation and some other education outcomes while older small schools do not. Importantly, we show that ignoring this source of treatment effect heterogeneity by assuming a common small school effect yields a misleading zero effect of small school attendance.