This quasi-experimental exploratory study investigated whether a social-emotional learning program, implemented during a 1-year period, could lead to gains in social-emotional competencies and a reduction in internalizing and externalizing problems. Furthermore, it showed which pupils would benefit most from the program. The program was applied to 213 fourth-grade Portuguese pupils. One hundred five controls followed an Origami curriculum during the same period. Sixteen teachers also participated in this study. Self-report (pupils) and hetero-report (teachers) questionnaires were administered before and after the intervention. There were significant intervention gains in some social-emotional competencies, namely, peer relations and social competence, but no gains were found in internalizing and externalizing problems. Intervention pupils with average pre-test scores profited more in self-management and peer relations than controls. Boys showed greater gains in self-management, aggressiveness, and social problems than girls. There were no significant differences regarding socioeconomic status. (Contains 4 tables.)