Better together? Heterogeneous effects of tracking on student achievement

I study the effects of early between-school ability tracking on student achievement, exploiting institutional differences between German federal states. In all states, about 40% of students transition to separate academic-track schools after comprehensive primary school. Depending on the state, the remaining student body is either directly tracked between two additional school types or taught comprehensively for another two years. Comparing these students before and after tracking in a tripledifferences framework, I find evidence for positive effects of prolonged comprehensive schooling on mathematics and reading scores. These are almost entirely driven by low-achievers. Early and rigid forms of tracking can thus impair both equity and efficiency of school systems.