Why do some students learn more in some schools than others? One consideration receiving growing attention is school management. To study this, researchers need to be able to measure school management accurately and cheaply at scale, and also explain any observed relationship between school management and student learning. This paper introduces a new approach to measurement using existing public data, and applies it to build a management index covering 15,000 schools across 65 countries, and another index covering nearly all public schools in Brazil. Both indices show a strong, positive relationship between school management and student learning. The paper then develops a simple model that formalizes the intuition that strong management practices might be driving learning gains via incentive and selection effects among teachers, students and parents. The paper shows that the predictions of this model hold in public data for Latin America, and draws out implications for policy.