This study examined the effects of peer groups on changes in academic engagement in 11- to 13-year-old children. From the entire cohort of 366 sixth graders in a town, 87% participated at the beginning and end of the school year. Peer groups were assessed using socio-cognitive mapping; as an indicator of motivation, teachers reported on students' classroom engagement. Peer groups were homogeneous in terms of engagement, and despite considerable member turnover across time, their motivational composition remained fairly intact. Peer group engagement levels in the fall predicted changes in children's motivation across time. Although the magnitude of effects was relatively small, evidence for group influences persisted when controlling for peer selection and the influence of teacher and parent involvement.