The impact of a school-based violence prevention program, Second Step, on peer victimization and aggression, and emotion regulation was evaluated among 457 sixth graders. A cluster-randomized trial was conducted with classrooms randomly assigned to intervention (n = 14) or control (n = 14) conditions. A repeated measures analysis of covariance on each measure was conducted using random effects. Several intervention effects were moderated by gender and disability status. Teacher-rated relational victimization at posttest decreased for students with disabilities in intervention but not control classrooms. Students without disabilities in intervention classrooms reported greater decreases from pretest to 6-month follow-up in overt aggression. Boys in the intervention classrooms had smaller increases in teacher-rated overt aggression at posttest than boys in control classrooms. Girls in intervention classrooms reported greater decreases from pretest to 6-month follow-up in relational aggression than girls in control classrooms. Study implications and directions for future research are discussed.