The Gatehouse Project is an innovative, comprehensive approach to mental health promotion in secondary schools. It sets out to promote student engagement and school connectedness as the way to improve emotional well-being and learning outcomes. The key elements of the whole-school intervention are the establishment and support of a school-based adolescent health team; the identification of risk and protective factors in each school's social and leaning environment from student surveys; and, through the use of these data, the identification and implementation of effective strategies to address these issues. The project evaluation used a cluster-randomized controlled trial design involving 26 schools with initial results demonstrating considerable success in reducing smoking rates among Year 8 children. This article describes and accounts for how system-level changes have been made in schools through a process of capacity building. This encourages teachers, parents, and students to view the core business of education differently.