Classroom teachers need effective, efficient strategies to prevent and/or ameliorate destructive student behaviors and increase socially appropriate ones. During the past two decades, researchers have found that cognitive strategies can decrease student disruption/aggression and strengthen pro-social behavior. Following preliminary pilot work, we conducted a study to determine whether a class-wide, social problem-solving curriculum affected measures of knowledge and behavior for 165 4th and 5th grade students at risk for behavior problems. We found significant positive treatment effects on knowledge of problem-solving concepts and teacher ratings of aggression. Outcomes differed across teachers/classrooms, and there was no evidence that booster lessons affected treatment efficacy. Teacher ratings of social validity were generally positive. We discuss issues about classroom-based prevention research and future research directions.