Abstract In this paper we evaluate the impact of grade span configuration on student outcomes. We build a model which shows that the more homogeneous the cohort of students the higher the share of them which achieves the minimum level of academic performance required in an exam. We then test this theoretical finding by comparing performances of 5th-grade students of elementary schools (1st to 5th grade) and those of elementary-middle schools (1st to 9th grade) in Brazil. PSM and PSM with Diff-in-Diff methodologies are used to control possible biases. We find that elementary schools present better results in Portuguese language and mathematics standardized score tests, higher passing rates and lower dropout rates than elementary-middle schools, which corroborates the theoretical results. The robustness of estimates is checked through several tests and alternative specifications. Finally, we found evidence that the main mechanisms behind our results are related to alternative pedagogical practices and school management policies.