Background: Physical activity (PA) declines as children and adolescents age. The purpose of this study was to examine how specific school factors relate to youth PA, TV viewing, and body mass index (BMI). Methods: A sample of 12- to 18-year-old adolescents in 3 cities (N = 165, 53% females, mean age 14.6 [plus or minus] 1.7 years, 44% nonwhite) completed surveys assessing days of physical education (PE) class per week, school equipment accessibility, after-school supervised PA, and after-school field access. Regression analyses were conducted to examine relationships between these school factors and PA at school facilities open to the public (never active vs active), overall PA level (days per week physically active for 60 minutes), BMI z score, and TV watching (hours per week). Results: Adjusting for demographics, days of PE per week and access to school fields after school were correlated with overall PA ([beta] = 0.286, p = 0.002, semipartial correlation 0.236 and [beta] = 0.801, p = 0.016, semipartial correlation 0.186, respectively). The association between after-school field access and overall PA was mediated by use of publicly accessible school facilities for PA. After-school supervised PA and school PA equipment were not associated with overall PA. In adjusted regression analyses including all school factors, days of PE remained correlated to overall PA independent of other school factors ([beta] = 0.264, p = 0.007, semipartial correlation = 0.136). There were no associations between school factors and BMI or TV watching. Conclusions: Based on these study findings, PE is a promising intervention to address improving overall adolescent PA within the school setting. (Contains 2 tables.)