Anger and aggressive behaviour are part of everyday school-life and have been reported to be negatively associated with student well-being and academic performance. School-based interventions, developed to target anger and aggressive behaviour, are scarce. One such Danish intervention, the so-called Mini-Diamond, has been developed and implemented in several Danish municipalities. The aim of the current study was to evaluate the potential effects of the Mini-Diamond on student well-being in a controlled trial. Outcome measures included aspects of child- and parent-rated student well-being, including school connectedness, learning self-efficacy, and bullying. All children in grades 0–2 in Rødovre (intervention) and Herlev (control) municipalities participated and filled out the same questionnaires. No effects of the Mini-Diamond intervention were found on any of the outcomes. Potential reasons for this are discussed, including the possibility that there is no effect of the Mini-Diamond intervention on well-being, together with limitations and implications of the current study.