This study investigated the effects of the Leader in Me (LiM), a social-emotional learning program implemented in thousands of schools. A mixed-method design included (1) a collective case study in four diverse, high-implementing elementary schools and (2) a post hoc quasi-experiment using propensity score 1:3 matching of 117 LiM and 348 non-LiM schools to compare achievement and discipline incident rates. The case study indicated that participants perceived LiM as improving social-emotional competencies for both teachers and students. Teachers reported being more prosocial, feeling more camaraderie, using more effective discipline, developing better teacher-student relationships, and enjoying teaching more. Students were more prosocial, confident and motivated, and had fewer problem behaviors. School climate improved. Results replicated across four diverse schools. The quasi-experiment supported results of the collective case study. Schools implementing LiM had higher mathematics and ELA, but not science, scores on state proficiency exams and fewer discipline incidents.