Objective: To understand the extent to which adolescent awareness about anaemia and anaemia prevention can be changed by nutrition messages received at school. Design: Mixed-methods pre–post intervention study. Setting: Three government schools in Bagalkot, Belagavi and Raichur districts of Karnataka, India. Population: Students of grade six and seven and teachers involved in implementing the intervention. Methods: An educational intervention was co-developed by school teachers and nutrition experts using locally adapted resource materials that consisted of lectures, role play and practical demonstrations. Seven half-hour educational sessions were delivered by school teachers over 7 weeks to 455 students. Pre- and post-intervention tests measured changes in adolescents' knowledge about anaemia. Semi-structured in-depth interviews with teachers and focus groups with students explored their reactions to the intervention. Main outcome measures: Knowledge score related to anaemia. Results: The percentage of children with correct scores increased by 7.3–49.0 percentage points for the tested questions after implementation of the intervention. The mean knowledge score increased by 3.67 ± 0.17 (p < 0.01). During interviews, teachers and students highlighted high acceptance of the intervention and materials, an increase in awareness, a positive attitude towards changing behaviour around diet, an increase in the demand for iron and folic acid supplements and improved sharing of messages learned with peers and families. Challenges expressed included need for further training, time limitations and hesitancy in teaching about menstruation and pregnancy. Conclusions: Educational interventions carried out for adolescents by teachers in schools are effective in improving awareness and attitude related to anaemia and its prevention.