Background: Body image dissatisfaction during adolescence is common but not benign. School-based interventions have the potential for wide reach, but scalability of previous programmes is limited by a reliance on external facilitators. Aims: To assess the acceptability, feasibility and efficacy of a teacher-delivered body image intervention. Method: A pilot clustered randomised controlled trial in which 16 classes of adolescent girls were allocated to a 6-session body image programme (n = 261), or usual curriculum control (n = 187) (registration: ISRCTN42594993). Results: Students in the intervention group had significantly improved body esteem and self-esteem and reduced thin-ideal internalisation. Effects for body esteem and thin-ideal internalisation were maintained for 3 months. There were no group differences for eating pathology, peer factors or depression. Acceptability, feasibility and efficacy varied between schools. Conclusions: Teacher-delivered body image lessons have promise but further work is needed to increase efficacy and make interventions suitable across a range of schools.