This study uses National Longitudinal Study of Adolescent Health (Add Health) data to explore the co-evolution of friendship networks and delinquent behaviors. Using a stochastic actor–based (SAB) model, we simultaneously estimate the network structure, influence process, and selection process on adolescents in 12 small schools (N = 1,284) and 1 large school (N = 976) over three time periods. Our results indicate the presence of both selection and influence processes. Moderating effects were tested for density, centrality, and popularity, with only a weak interaction effect for density and influence in the small schools (p < .10). Contexts outside the school affected school networks: adolescents in the large school were particularly likely to form ties to others from equally disadvantaged neighborhoods, and adolescents in the small schools with more outside of school ties increased their delinquency over time. These findings support the importance of delinquency in peer selection and influence processes.