Evidence suggests that mental health interventions are more effective when they consider the whole context of schools; addressing the needs of all students, their families, and staff; otherwise known as a whole-school approach (WSA). The UK Government is piloting WSAs to transform mental health and wellbeing by locating educational mental health practitioners in educational settings across England. This study aims to develop a ‘bottom-up’ understanding of the contextual factors and mechanisms that underlie WSAs in Trailblazer schools in the North East and North Cumbria, to gain insight into the facilitators and barriers of delivering a WSA, and optimal evaluation methods. To undertake a realist evaluation, we included the generation of initial programme theories from existing academic literature and policy documents; ‘theory gleaning’ interviews with NHS/local authority stakeholders, Trailblazer staff and school senior leaders; refining and development of theories; and individual interviews and focus groups with pupils, parent/carers and school staff. The findings will enable Trailblazer partners to better understand how their WSAs to mental health contain the essential components for transformation in schools in the region. This will contribute to the embedding of continuous evaluation into regional Trailblazers’ practice for participating schools, for subsequent annual waves and producing relevant findings for non-Trailblazer schools. Complementing the national evaluation of all 25 Wave 1 Trailblazer pilot sites, this study will generate an explanatory theoretical account of how to optimally design, implement and evaluate WSAs by exploring the contextual factors associated with implementation of WSAs.