Most children do not engage in enough physical activity and spend a significant portion of their school days in sedentary behaviours (SB). We explored how different time segments of the day contribute to daily levels of SB, light-intensity physical activity (LPA), and moderate-to-vigorous physical activity (MVPA) among Canadian school children. Students (n = 193; 50.3% male, average age 9.3 years) from three elementary schools wore accelerometers for 7-8 days. School schedules were used to estimate behaviours during specific time segments: Before school (06:00-08:44), in-school time (08:45-15:04), after school (15:05-16:59), and evening (17:00-21:59). Children engaged in an average of 581.0±74.8 minutes/day of SB (65.5% of the day), 200.4±48.7 minutes/day of LPA (22.5% of the day), and 107.4±45.5 minutes/day of MVPA (12.0% of the day). Children spent 63.4% of the school day in SB, 23.3% in LPA, and 13.3% in MVPA. However, in-school time accounted for 38.0% of daily SB, 40.7% of LPA, and 44.7% of MVPA. The shorter after-school period was the most active segment but contributed only 16.2% of daily MVPA, 16.7% of LPA, and 12.0% of SB. The evening segment contributed to 29.6% of daily MVPA, 28.6% of LPA, and 27.4% of SB. In conclusion, given the amount of time spent at school, its impact on children's MVPA could be optimized. The results highlight the need for tailored theory-based interventions for the school and after-school periods aiming at empowering children to move more and sit less.