This study examined longitudinal changes in peer network selection and influence associated with self-reported antisocial behavior (AB) and violent behavior (VB) over the course of middle school in a sample of ethnically diverse adolescents. Youth and families were randomly assigned to a school-based intervention focused on the prevention of substance use. Using a longitudinal social network modeling approach, we found that students with similar levels of AB were more likely to affiliate during the transition from sixth to seventh grade (peer selection). Higher levels of AB in the seventh grade led to growth in network size by eighth grade. No peer selection effects documented for VB, but youth became more similar to total levels of VB reported by their peers between seventh and eighth grades (peer influence). As expected, AB was positively associated with VB levels. Random assignment to intervention condition affected peer selection at Grade 7 but did not reduce levels of self-reported AB or VB. These findings suggest that managing peer networks using a variety of strategies is likely to reduce peer contagion for AB and VB in a public middle school setting. We discuss emerging intervention strategies for the management and the prevention of emotional and behavioral disorders.