Background: Schools should provide children 30 minutes/day of moderate-to-vigorous-physical-activity (MVPA). Determining school day segments that contribute to children's MVPA can inform school-based activity promotion. The purpose of this paper was to identify the proportion of children accumulating 30 minutes/day of school-based MVPA, and to identify school day segments' contribution to children's MVPA. Methods: First-third graders (N = 323 [173 girls and 150 boys]) accelerometer-derived sedentary behaviors, light activity, and MVPA were measured for 1 to 5 school days. Children's activity was estimated during class time, lunch, physical education (PE), and recess. To explore disparities in MVPA, children were stratified into high/low-active groups. Results: Girls and boys accumulated 30.4 and 34.4 minutes/day of MVPA on PE days and 23.5 and 27.3 minutes/day of MVPA on non-PE days. This translated to 36.5 and 44.1% of girls and boys accumulating 30 minutes of MVPA on PE days and 16.3 and 36.5% on non-PE days. On PE days, PE contributed the most MVPA for girls and boys (11.8 and 13.0 minutes/day), followed by class time (11.4 and 12.5 minutes/day), recess (5.2 and 6.5 minutes/day), and lunch (2.0 and 2.4 minutes/day). Disparities between high/low children were greatest during PE and class time. Conclusions: Most children were not accumulating 30 minutes/day of MVPA. PE and class time are promising school day segments for promoting MVPA.