In recent decades, an international consensus has grown on incorporating social-emotional learning (SEL) in educational curricula. Its long-term benefits for children and youth are well-known, including improved mental health, social functioning and academic performance. Teachers are essential for promoting SEL in schools. However, they often lack the proper training and tools to implement SEL practices. Attending to the need to provide professional development opportunities for teachers regarding SEL, we present an in-service teacher training program for promoting social-emotional skills and positive social relationships in secondary schools. We use mixed methods to evaluate its potential in a pilot study with 113 Uruguayan teachers. A quasi-experimental design evaluates the program’s impact on teacher self-efficacy, personal accomplishment, and positive teacher-student relationships, which are considered bases for effective SEL practices. Focus groups with 53 program participants provide qualitative insights regarding the program’s contributions to teachers’ perceived confidence to promote SEL. Results show a significant positive effect of the program on teacher-perceived teacher-student relationships and sense of personal accomplishment through teaching. These changes are not observed in the control group. Qualitative data reveal that the opportunities for individual and collective reflection promoted through the program allow for the de- and reconstruction of one’s educational practices and institutional dynamics in favor of SEL. Conclusions are drawn for future programs that promote SEL and positive social relationships in schools.