Background: While school-based anti-bullying programs are widely used, there have been few controlled trials of effectiveness. This study compared the effect of manualized School Psychiatric Consultation (SPC), CAPSLE (a systems and mentalization focused whole school intervention), and treatment-as-usual (TAU) in reducing aggression and victimization among elementary school children. Method: Participants were 1,345 third to fifth graders in nine elementary schools in a medium-sized Midwestern city who took part in a cluster-level randomized controlled trial with stratified restricted allocation, to assess efficacy after two years of active intervention and effectiveness after one year of minimal input maintenance intervention. Outcome measures included peer and self-reports of bullying, bystanding, and mentalizing behavior and classroom behavioral observations of disruptive and off-task behavior. Results: CAPSLE moderated the developmental trend of increasing peer-reported victimization (p less than 0.01), aggression (p less than 0.05), self-reported aggression (p less than 0.05) and aggressive bystanding (p less than 0.05), compared to TAU schools. CAPSLE also moderated a decline in empathy and an increase in the percent of children victimized compared to SPC (p less than 0.01) and TAU conditions (p less than 0.01). Results for self-reported victimization, helpful bystanding, and beliefs in the legitimacy of aggression did not suggest significantly different changes among the study conditions over time. CAPSLE produced a significant decrease in off-task (p less than 0.001) and disruptive classroom behaviors (p less than 0.01), while behavioral change was not observed in SPC and TAU schools. Superiority with respect to TAU for victimization (p less than 0.05), aggression (p less than 0.01), and helpful (p less than 0.05) and aggressive bystanding (p less than 0.01) were maintained in the follow-up year. Conclusions: A teacher-implemented school-wide intervention that does not focus on disturbed children substantially reduced aggression and improved classroom behavior.