There is evidence that universal school-based mindfulness training (SBMT) can have positive effects for young people. However, it is unknown who benefits most from such training, how training exerts effects, and how implementation impacts effects. This study aimed to provide an overview of the evidence on the mediators, moderators, and implementation factors of SBMT, and propose a conceptual model that can be used both to summarize the evidence and provide a framework for future research. A scoping review was performed, and six databases and grey literature were searched. Inclusion and exclusion criteria were applied to select relevant material. Quantitative and qualitative information was extracted from eligible articles and reported in accordance with PRISMA-ScR guidelines. The search produced 5479 articles, of which 31 were eligible and included in the review. Eleven studies assessed moderators of SBMT on pupil outcomes, with mixed findings for all variables tested. Five studies examined the mediating effect of specific variables on pupil outcomes, with evidence that increases in mindfulness skills and decreases in cognitive reactivity and self-criticism post-intervention are related to better pupil outcomes at follow-up. Twenty-five studies assessed implementation factors. We discuss key methodological shortcomings of included studies and integrate our findings with existing implementation frameworks to propose a conceptual model. Widespread interest in universal SBMT has led to increased research over recent years, exploring who SBMT works for and how it might work, but the current evidence is limited. We make recommendations for future research and provide a conceptual model to guide theory-led developments.