Children with high levels of aggressive behavior create a major management problem in school settings and interfere with the learning environment of their classmates. We report results from a group-randomized trial of a program aimed at preventing aggressive behaviors. The purpose of the current study, therefore, was to determine the extent to which an indicated prevention program, Coping Power Program, is capable of reducing behavioral problems and improving pro-social behavior when delivered as a universal classroom-based prevention intervention. Nine classes (five first grade and four second grade) were randomly assigned to intervention or control conditions. Findings showed a significant reduction in overall problematic behaviors and in inattention-hyperactivity problems for the intervention classes compared to the control classes. Students who received Coping Power Program intervention also showed more pro-social behaviors at postintervention. The implications of these findings for the implementation of strategies aimed at preventing aggressive behavior in school settings are discussed.