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 implementation quality impacts effects, and how training exerts effects. No known scoping reviews have comprehensively reviewed moderation, mediation and implementation factors of SBMT. 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 based on methodology outlined by the Joanna Briggs Institute’s Manual. 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 5, 479 articles, of which 37 were eligible and included in the review. Eight studies assessed moderators of SBMT on pupil outcomes, with mixed findings for all variables tested. Four studies examined the mediating effect of specific variables on pupil outcomes, with evidence that increases in trait mindfulness 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 of SBMT, 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 and research in SBMT.