Background: This study examined the distribution of objectively measured physical activity (PA) and sedentary time of fifth-grade students during school, leisure time, and physical education (PE) classes. Demographic, anthropometric, and PA data were collected from 17 representative Finnish schools. Methods: To estimate the PA and sedentary time, participants (N = 592) wore wGT3X-BT ActiGraphs for 7 consecutive days. Comparisons were made between genders and different BMI groups. Results: From the study sample, 43.7% met the moderate to vigorous PA (MVPA) guidelines. Participants spent 62.2% of the day sedentary and 8.2% in moderate and vigorous activities. Boys performed more MVPA than girls, and girls were more sedentary during school days. Boys had more MVPA than girls in leisure time, but there were no differences in sedentary time. However, an examination of PA assessed during PE classes revealed no differences between boys and girls. Normal-weight boys engaged in more MVPA than overweight and obese boys. No differences were found for girls. Conclusions: The PA levels differ between different BMI groups in leisure time and during school but not during PE lessons. PA for overweight children should be targeted and compulsory PE time should be increased to achieve the PA guidelines.