Background: Sedentary behavior (SB) is related to unhealthy outcomes and is performed in many contexts, including school. The aim of this study was to identify sociodemographic, biological, and psychosocial correlates of SB performed at school in a sample of adolescents. Methods: Adolescents provided information regarding sex, age, socioeconomic status, and psychosocial variables related to physical activity (self-efficacy, attitudes, perception of school environment, peer and parental support). Body fat was estimated using skinfolds, and maximum aerobic capacity with a 20 m shuttle run. Adolescents wore accelerometers and SB performed during class time, physical education (PE) classes, and recesses was estimated. Results: Adolescents (N = 567, 53% female, 12.9 ± 5.3 years old) spent 67% of class time, 41% of PE classes, and 41% of recesses in SB. Girls spent more time in SB, and age was negatively associated with SB. Body fat was positively associated with SB in class time and PE classes. Peer support for physical activity was negatively associated with SB in class time, whereas attitudes related to physical activity were negatively associated with SB during PE classes and recesses. Conclusion: Adolescents spend long periods in SB at school, even during opportunities to be active like PE classes and recesses.