The education sector is an important component of the UK's net zero strategy, in terms of both the carbon footprint of school buildings and operations, and the opportunities to teach about environmental issues and empower climate action. However, school sustainability is often narrowly defined around individual choices and behaviours by different school stakeholders, rather than the broader patterns of social practices. This qualitative study of secondary schools in England involved stakeholder interviews and student focus groups at twelve schools in Greater London and the Thames Valley Region (Berkshire, Buckinghamshire, Oxfordshire) where 142 people participated in this research, including teachers, students, parents, governors and school staff (leadership, facilities, finance). School sustainability was explored through the lens of social practice theory, and three bundles of practices and arrangements were identified: teaching/learning, catering/eating, movement/travel. Whole-school approaches to sustainability were reframed through the semantic, material and social spaces identified in the theory of practice architectures. School sustainability requires a substantial investment into retrofitting school buildings, but it also needs to be woven in the culture of a school – firmly on the agenda of the governors and leadership team, parallel to an issue like safeguarding – and supported by clearly identified roles and relationships in each institution. This research is aimed at environmental educators and researchers wishing to apply insights from social theory to develop more effective whole-school approaches to sustainability. This research also reveals a potential divide between state and private schools when it comes to environmental education, empowerment and action.