Effectiveness of Mental Health Promotion in Primary Schools: Results of a Cluster-Randomized Controlled Trial

Background: This study evaluated the effectiveness of the mental health promotion program MindMatters in primary schools. Methods: A cluster randomized controlled trial (2021–2023) included 37 German primary schools (18 intervention, 19 control). Pupils in grades 1–3 (ages 6–9) and their teachers were surveyed before and after implementation. Data from 2896 pupils were analyzed, covering mental health (SDQ), social–emotional and academic skills, and learning behavior. Results: A three-factor SDQ model (externalizing, internalizing, prosocial) was applied. At follow-up, no significant differences were found between the intervention and control groups in pupils' self-reported mental health or in teachers' reports of pupils' mental health. Likewise, there were no significant differences in children's self-reported social–emotional skills or in academic outcomes reported by the teachers at follow-up. Implications for School Health Policy, Practice, and Equity: Schools should invest in evidence-based programs to strengthen social interaction and learning environments. Primarily, education and health policy should create and mandate conditions that facilitate evidence generation for whole-school interventions, while ensuring that schools receive the support needed for effective implementation. Conclusions: This study provides important evidence for complex mental health promotion programs. Future studies should consider longer implementation periods and higher program uptake. Trial Registration: German Clinical Trials Register: DRKS00023762.