Policy change, in the form of mandatory improvements in nutrition standards for foods served as part of school meals, has a substantial impact on the nutrient and energy density of foods served in secondary schools. Using prospective data from separate studies in three school districts, we show that the improvements in nutritional quality of students’ food choices associated with mandatory nutrition standards are of greater magnitude than changes nutritional quality associated with classroom nutrition education, menu labeling, choice architecture/behavioral economics interventions in the cafeteria, and social marketing approaches. National policy change is a powerful force for improving school nutrition and can serve as a foundation for additional initiatives to improve children’s health.