Objective. Physical education (PE) during school provides an opportunity for children to be physically active. Few empirical studies have investigated developmental trajectories and determinants of objective moderate to vigorous physical activity (MVPA) during PE classes. The purpose of this study was to examine the developmental trajectories and determinants of MVPA during PE lessons in young children (8–12 years of age) in primary schools. Methods. Students in grades 5–7 (n=1202; 51.2% girls) were recruited from 17 elementary schools from the Geneva canton in 2012–2013. The percentage of time spent in accelerometer-assessed MVPA during regular PE lessons was used as a dependent variable. Results. Linear mixed-model analyses revealed (a) that boys had a higher percentage ofMVPA than girls, but none of the children reached the recommended activity levels (i.e., 50% of the PE class time spent inMVPA), (b) a linear decrease in the percentage of MVPA with age, (c) that higher perceived competence predicted a higher percentage of MVPA, and (d) that higher perceived competence reduced the negative linear effect of age among boys, but not among girls. Conclusion. The percentage of PE time spent in MVPA did not reach recommendations made by Centers for Disease Control and Prevention and decreased from 8 to 12 years old both for boys and girls. Perceived competence appears crucial to reduce MVPA decline for boys, but not for girls.