Class-size reduction policies and the quality of entering teachers

• Prior studies find little evidence of achievement gains from class-size policies.
• We study whether a fall in new teacher quality can explain these results.
• Using pre- and post-policy data, we find a general decline in cohort value-added.
• The quality drop was short lived due to attrition by the lowest performers.
• All schools saw a drop, so it cannot explain prior treatment-control policy effects.

Class-size reduction (CSR) policies have typically failed to produce large achievement gains. One common explanation is that CSR forces schools to hire low-quality teachers. Prior studies of this hypothesis have been hindered by poor data. Using different data, we find that hiring quality did fall with state-wide CSR. However, this drop was temporary due to attrition by the lowest performers. Furthermore, the drop was similar for schools classified as treated and control for prior evaluations of CSR. Therefore, differences in the quality of incoming teachers cannot explain the estimated performance of CSR. This is consistent with hiring spillovers in connected markets.