In this article, we evaluate the current state of universal school-based mental health prevention. We argue that the field is at another pivotal turning point, with many unanswered questions. As youth mental health issues rise in prominence and prevalence, schools increasingly adopt mental health and well-being programmes. However, recent large-scale trials have reported varied effects, raising questions about their effectiveness. We critically analyse these findings and place them in a global context. We contend results in high-income countries should not be seen as a failure of universal prevention overall and unpack key learnings from trials that have not found significant preventive effects. A comprehensive perspective of the data reveals both negative and positive aspects, as well as some signposts for the way forward. We advocate for programme adaptation based on feedback, embracing proportionate universalism and exploring alternative prevention strategies without discarding universal prevention. We urge continued research with stakeholder involvement, emphasising a nuanced approach to universal school-based mental health prevention, is urgently needed to move the field forward.