This is an effectiveness study into a program (HiRO) for improving social-emotional development, self esteem and the perceived classroom peer context in primary school students (aged 4-13 years). To this end, schools are divided into three conditions based on the choice of the schools (non-randomized): 1) School As Usual, 2) HiRo without judo classes, and 3) HiRO with judo classes. In all participating schools, social-emotional skills, self-image, emotional problems, and classroom peer context are measured three times by means of questionnaires (both self-report and parent-report). In The Netherlands primary schools are obliged to offer students training in social-emotional development. Schools can decide to develop their own program or make use of existing programs delivered by third parties, such as HiRO. In this study HiRO is compared to school as usual, that is, any other program offered to promote social -emotional development than HiRO. The main questions to answer are: * What is the effect of HiRO on the development of prosocial behavior? * What is the effect of HiRO on the development of emotional problems (depression, anxiety)? * What is the effect of HiRO on self-esteem? * What is the effect of HiRO on perceived peer context? Researchers will compare HiRO with and without judo to school as usual testing the following hypotheses: HiRO will result in increased prosocial skills as compared to school as usual. HiRO will result in decreased emotional problems as compared to school as usual. HiRO will result in increased self-esteem as compared to school as usual. HiRO will result in improved perceived peer context as compared to school as usual.