Purpose: Evidence from the USA/Australia suggests whole-school interventions designed to increase social inclusion/engagement can reduce substance use. Completeness of implementation varies but contextual determinants have not been fully explored. Informed by previous interventions, the paper aims to examine these topics in an English pilot of the Healthy School Ethos intervention. Design/methodology/approach: This intervention, like previous interventions, balanced standardization of inputs/process (external facilitator, manual, needs-survey and staff-training delivered over one year to enable schools to convene action-teams) with local flexibility regarding actions to improve social inclusion. Evaluation was via a pilot trial comprising: baseline/follow-up surveys with year-7 students in two intervention/comparison schools; semi-structured interviews with staff, students and facilitators; and observations. Findings: The intervention was delivered as intended with components implemented as in the USA/Australian studies. The external facilitator enabled schools to convene an action-team involving staff/students. Inputs were feasible and acceptable and enabled similar actions in both schools. Locally determined actions (e.g. peer-mediators) were generally more feasible/acceptable than pre-set actions (e.g. modified pastoral care). Implementation was facilitated where it built on aspects of schools' baseline ethos (e.g. a focus on engaging all students, formalized student participation in decisions) and where senior staff led actions. Student awareness of the intervention was high. Originality/value: Key factors affecting feasibility were: flexibility to allow local innovation, but structure to ensure consistency; intervention aims resonating with at least some aspects of school baseline ethos; and involvement of staff with the capacity to deliver. The intervention should be refined and its health/educational outcomes evaluated. (Contains 3 tables.)