Purpose: Body dissatisfaction is common during adolescence and predicts poor psychological and physical health. Interventions have traditionally over-relied on delivery by external providers (e.g., researchers and psychologists), preventing scalability. This study evaluated the acceptability and effectiveness of a school-based body image intervention delivered by schoolteachers. Methods: Six British schools participated in a pragmatic cluster randomised controlled trial. Boys and girls aged 11-13 received the 5-session intervention delivered by their teachers (n=848), or lessons-as-usual control (n=647), and were assessed at baseline, post-intervention, 2-, 6-, 12-, 24- and 36-month follow-up. The primary outcome was body image (body esteem), secondary outcomes included risk factors for body image (internalisation of appearance ideals, sociocultural pressures, social comparisons, appearance-related teasing and conversations), and tertiary outcomes included psychosocial well-being (negative affect, self-esteem, dietary restraint, life engagement). Results: Compared to the control group, intervention students demonstrated improvements in the primary outcome of body esteem at post-intervention (Cohen’s d = 0.15), 2-month (d = 0.26), and 6-month follow-up (d = 0.15). For girls, there was also a significant reduction in experienced appearance-related teasing at 6-month (d = 0.24) and 12-month (d = 0.30) follow-up. No other significant intervention effects were observed. The intervention was moderate-highly acceptable to students. Conclusions: These findings present the longest sustained improvements in a cognitive affective body image outcome observed among girls and boys during a teacher-led universal body image programme to date. Intervention refinement and improved teacher training may further improve outcomes. Task-shifting intervention delivery to community providers to scale up interventions is a promising strategy.