A multisite cluster randomized trial was conducted to examine the effects of the Social Skills Improvement System Classwide Intervention Program (SSIS-CIP; Elliott & Gresham, 2007) on students' classroom social behavior. The final sample included 432 students across 38 second grade classrooms. Social skills and problem behaviors were measured via the SSIS rating scale for all participants, and direct observations were completed for a subsample of participants within each classroom. Results indicated that the SSIS-CIP demonstrated positive effects on teacher ratings of participants' social skills and internalizing behaviors, with the greatest changes occurring in classrooms with students who exhibited lower skill proficiency prior to implementation. Statistically significant differences were not observed between treatment and control participants on teacher ratings of externalizing problem behaviors or direct observation.