Education researchers, policy makers, and practitioners have emphasized the role that social-emotional learning and self-regulation play in children's adjustment and connection to school, particularly as they transition from pre-school to kindergarten and the primary grades. A pretest-posttest cluster-randomized efficacy trial of the Social-Emotional Learning Foundations (SELF) curriculum for kindergarten-first-grade students found positive main effects on assessments of self-regulation, social-emotional learning, social-emotional vocabulary, and general behavioral functioning. This study is a secondary analysis using structural equation modeling to explore whether SELF effects on school adjustment were mediated by its effects on language and/or self-regulation-related outcomes. Findings replicated direct effects of treatment but did not support hypothesized mediators. In contrast, direct effects of treatment on measures of competent school functioning and internalizing behavior were mediated by outcome effects on a standardized measure of social-emotional learning competence. Study findings underscore the fundamental importance of social-emotional learning to school success and suggest related measurement issues in social-emotional learning and topics for further research.