BACKGROUND: This paper argues for giving explicit attention to the quality of implementation of school-wide mental health promotions and examines the impact of implementation quality on academic performance in a major Australian mental health initiative. METHOD: Hierarchical linear modelling was used to investigate change in standardised academic performance across the 2-year implementation of a mental health initiative in 96 Australian primary (or elementary) schools that was focused on improving student social-emotional competencies. RESULTS: After controlling for differences in socioeconomic background, a significant positive relationship existed between quality of implementation and academic performance. The difference between students in high- and low-implementing schools was equivalent to a difference in academic performance of up to 6 months of schooling. KEY PRACTITIONER MESSAGE: Given the known relationship between student academic achievement and mental health, many nations are mounting school-based mental health interventions: however, the quality of program implementation remains a concernThe Australian KidsMatter primary school mental health intervention enabled the development of an Implementation Index allowing schools to be grouped into low- to high- implementing schoolsThe quality of implementation of KidsMatter appears to be positively associated with the level of student academic achievement, equivalent to 6 months more schooling by Year 7, over and above any influence of socioeconomic background.