Incidents of aggression are frequent in schools. In this paper a program aimed at improving relationships in the school is described and evaluated. A developmental view of cognitive and affective foundations of moral behavior was the theoretical basis for the program. The intervention comprised of three learning modules: interpersonal problem-solving, basic human values and emotional self-control. One teacher conducted the intervention in the classroom with her 30 first-grade students. Pre and post intervention assessments comprised prosocial performance assessed by peers, and perception of school stressors. Interpersonal conflicts were recorded in a field diary. Results showed that conflicts were reduced and prosocial performance improved after the intervention. Compared to students who had not received the intervention, the children showed better prosocial performance and less susceptibility to stressors after the intervention. The program was shown to have contributed to improving relationships among children. The study has implications for the prevention of aggression in the schools.