Social-emotional learning (SEL) and character education are important components of adolescent development. In this study, we evaluated a randomized controlled trial of CharacterStrong, a curriculum that combines SEL and character education that included 1609 students and 242 teachers across 14 schools. This study applied baseline target moderation analysis to examine the differential effect of CharacterStrong given students' baseline (pre-test) scores on self-reported measures of social-emotional competence and personal attributes (i.e., Self-efficacy, Grit, Learning strategies, Growth mindset, Self-management, and Social-awareness). Results indicated that students with the most room to improve at baseline (pre-test) who received CharacterStrong improved significantly more than those who also had the most room to improve in the control group for three out of the six measured outcomes: Self-efficacy, Learning strategies, and Self-management. Teachers who received training for CharacterStrong reported improvements in their own Self-efficacy and students' SEL competencies compared to teachers in the control condition. This study provides evidence supporting the effects of "CharacterStrong" as well as adds to the school-based prevention research on performing moderation analyses of baseline status to reveal the sub-group of youth who respond most favorably to universal prevention programs.