Research has consistently linked social-emotional skills to important educational and life outcomes. Many children begin their school careers, however, without the requisite social and emotional skills that facilitate learning, which has prompted schools nationwide to adopt specific curricula to teach students the social-emotional skills that enable them to maintain optimal engagement in the learning process. Second Step® is one of the most widely disseminated social-emotional learning (SEL) programs; however, its newly revised version has never been empirically evaluated. The purpose of this study was to conduct a randomized controlled trial investigating the impact of the 4th Edition Second Step® on social-behavioral outcomes over a 1-year period when combined with a brief training on proactive classroom management. Participants were kindergarten to 2nd grade students in 61 schools (321 teachers, 7300 students) across six school districts. Hierarchical models (time×condition) suggest that the program had few main effects from teacher-reported social and behavioral indices, with small effect sizes. The majority of significant findings were moderated effects, with 8 out of 11 outcome variables indicating the intervention-produced significant improvements in social-emotional competence and behavior for children who started the school year with skill deficits relative to their peers. All the significant findings were based on teacher-report data highlighting a need for replication using other informants and sources of data. Findings provide program validation and have implications for understanding the reach of SEL programs.