BACKGROUND: This study assessed the association of school environmental characteristics with student physical activity on campus. METHODS: Physical activity areas (n = 137) at 24 public middle schools were assessed for area type, size, and improvements (e.g., basketball courts). Student physical activity and the presence of equipment and supervision were directly observed before school, after lunch, and after school. RESULTS: Environmental characteristics explained 42% of the variance in the proportion of girls who were physically active and 59% of the variance for boys. CONCLUSIONS: School environments with high levels of supervision and improvements stimulated girls and boys to be more physically active.