The purpose of this study was to explore the effectiveness of a universal classroom-based social-emotional learning (SEL) program for first grade students. Forty classrooms from 13 elementary schools across 3 states participated in the trial. Teachers in classrooms randomly assigned to the implementation condition were provided with free access to all curricular materials from the SSIS SEL Classwide Intervention Program (Elliott & Gresham, 2017). Consistent with the goals of an effectiveness trial, teachers made local decisions regarding how they implemented the SSIS SEL CIP within their own classroom. Measures of participating students' (N = 365) social behavior (positive and negative) were collected before and after program implementation in all participating classrooms (treatment and business-as-usual control). Overall, results were mixed relative to hypotheses. Main effects tended to be non-significant and negligible in size; however, several interactions indicated positive outcomes resulting from program exposure (increases in prosocial behavior, decreases in negative social behavior) for students with lower skills at baseline. In addition to replication, future research directions include identifying critical program components and approaches to implementation that optimize program effectiveness under authentic conditions.