The National Study of Learning Mindsets evaluated a short, online growth mindset intervention in a nationally representative sample of secondary education students in the United States. The intervention, which teaches that intellectual abilities can be developed, improved grades among lower-achieving students and increased enrollment in advanced mathematics courses. The study identified school contexts that sustained these effects, particularly when peer norms aligned with the intervention's messages. The intervention was effective in reducing fixed mindset beliefs and improving GPA, with larger effects in schools with supportive peer norms and lower achievement levels. A Bayesian analysis confirmed the positive average treatment effects. The intervention also increased the likelihood of students taking advanced mathematics courses, with stronger effects in schools with supportive peer norms. This study highlights the importance of both belief change and school environments in enhancing academic outcomes and suggests a new era of interdisciplinary research to understand and enhance adolescent development.The National Study of Learning Mindsets evaluated a short, online growth mindset intervention in a nationally representative sample of secondary education students in the United States. The intervention, which teaches that intellectual abilities can be developed, improved grades among lower-achieving students and increased enrollment in advanced mathematics courses. The study identified school contexts that sustained these effects, particularly when peer norms aligned with the intervention's messages. The intervention was effective in reducing fixed mindset beliefs and improving GPA, with larger effects in schools with supportive peer norms and lower achievement levels. A Bayesian analysis confirmed the positive average treatment effects. The intervention also increased the likelihood of students taking advanced mathematics courses, with stronger effects in schools with supportive peer norms. This study highlights the importance of both belief change and school environments in enhancing academic outcomes and suggests a new era of interdisciplinary research to understand and enhance adolescent development.