Two samples of sixth-grade students were followed over time to examine relations of number of reciprocated friendships, peer acceptance, and group membership to academic achievement. In both samples, group membership was the most consistent predictor of grades over time. In Study 2, prosocial behavior, antisocial behavior, and emotional distress were examined as processes that might explain these significant links between peer relationships and academic achievement. Results of longitudinal analyses support a conclusion that aspects of peer relationships are related to classroom achievement indirectly, by way of significant relations with prosocial behavior. Future research might benefit from more in-depth analyses of the functions of adolescent peer relationships and the processes by which they influence orientations toward social and academic competence at school.