Class size and school gender composition

Abstract This study examines the effect of class size on the academic achievement of ninth‐grade students, using random variation in enrollment as an instrument for class size. I find a substantial improvement in reading test scores after a reduction in class size. These favorable effects are especially strong for students in coeducational schools and for boys. Distributional analysis shows a beneficial effect on reading across all percentiles and on mathematics and English in the lower and middle percentiles. I find that reducing class sizes increases counseling sessions and reduces incidents of school violence, resulting in beneficial effects on academic performance. These findings indicate that the beneficial effect can be attributed to the improved teacher–student relationship and the peer‐to‐peer relationship. These results suggest that the effect of class size reduction depends on the classroom environment.