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.