The aim was to assess the relationship between school rhythm and physical activity (PA) in adolescents. The study included 2024 adolescents (12.5-17.4years). Participants wore an accelerometer for 7days. A short school rhythm was defined as a short time at school with short recesses and less time in teaching per day (Group 1). A long school rhythm was defined as a longer time at school with more time in teaching and recess (Group 2). Adolescents in Group 1 performed less moderate to vigorous PA (MVPA) than those in Group 2 per week (P<.0001), especially during school days (recess: 3.9 +/- 4.0 vs. 9.8 +/- 7.9minday(-1); P<.0001; teaching hours: 14.5 +/- 9.8 vs. 19.1 +/- 12.0minday(-1); P<.0001). Adolescents in Group 1 were less likely to meet the PA recommendations than were adolescents in Group 2: 30.7% vs. 34.1% (P<.0001). During school days, the percentage of adolescents who spent more than 2hday(-1) in sedentary activities was greater in the Group 1 (P<.001). Our results suggest that leisure-time out-of-school hours is used mainly for sedentary activities, and that school time provides a good opportunity for promoting PA.