There is widespread agreement that schooling quality should be a priority in the post-2015 education agenda, but less agreement on how quality can be enhanced in a cost effective manner. In Uganda, classroom overcrowding is often considered a critical cause of poor learning outcomes. This paper investigates how various aspects of classroom composition, including class size and the achievement distribution of classmates, affect individual learning. Using test score data for over 250,000 children, such compositional factors are found to be relevant. However, their influence on learning is not so large as to justify major standalone policy interventions. Rather, pedagogical changes that support children with literacy difficulties merit particular consideration.