The proposed study is a qualitative strand embedded in a two-arm (intervention vs. control) parallel cluster randomised controlled trial design, with mainstream primary schools across the Greater Manchester city region and surrounding areas. We will use a qualitative longitudinal design, engaging with children, teachers, and senior leaders in five case study schools via focus groups and interviews. Data from all sources will be analysed collectively using reflexive thematic analysis to build an integrated picture. Analysis will be a blend of deductive and inductive, guided by relevant implementation theory on dimensions of implementation and factors affecting implementation, and we will triangulate and examine differences in thematic representation among the different participant groups to understand differences in perspectives and insights.