This study examined whether peer groups can indirectly affect children's academic development in sixth grade (ages 11 to 13) by influencing their engagement in the classroom. From the entire cohort of 366 sixth graders in a town, 87% provided information 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. Achievement scores were averages of students' grades (summed across performance and effort scores in reading, writing, and mathematics) and achievement scores. Levels of classroom engagement within children's peer groups in the fall predicted children's own motivation at the same time,and, in turn, children's individual motivation scores predicted changes in their achievement over the school year. Implications are discussed for the cumulative effects of children's peer groups on their academic development during middle school.