The purpose of this study was to assess the physical activity levels of schoolchildren during physical education lessons, using heart rate telemetry. Girls (n = 108) and boys (n = 69), age 9 to 15 years, were assessed over 66 physical education lessons. Lessons that achieved a heart rate (HR) of ≥ 150 bpm for 20 minutes or 50% of lesson time were deemed sufficiently active to promote cardiorespiratory fitness. Netball, 11- to 12-year-old girls’ gymnastics, and soccer lessons achieved the HR ≥ 150 level for over 50% of lesson time. A Sex × Age × Lesson Activity ANOVA indicated significant overall interaction for sex, age, and lesson activity for the percentage of lesson time spent in HR ≥ 150. Significant differences between age groups and lesson activities were evident. Invasion games seem more likely to attain MVPA goals than are dance, track and field, fitness, or gymnastics lessons.