The aim of the present study was to examine objective levels of physical activity during Physical Education (PE) classes in Chilean adolescents, identifying possible differences according to gender. A sample composed of 156 Chilean adolescents (69 female and 87 male, average age = 13.5 ±.7 years) participated in this study. A total of 34 adolescents did not meet the inclusion criteria and were excluded, with 122 participants finally considered for the data analyses. Objective levels of physical activity were assessed by means of GT3X accelerometers during a PE lesson. The results showed that none of the participants reached the recommended 50% of total PE time at moderate-to-vigorous physical activity level. Most of the students (50.7% males and 72.5% females) accumulated less than 10% of the PE time at moderate-to-vigorous physical activity level. Boys showed higher values of moderate-to-vigorous physical activity than girls (p <.05); however, there were no gender differences in sedentary behavior or light physical activity time (p >.05). Chilean adolescents do not meet the moderate-to-vigorous physical activity recommendations during PE lessons. Different strategies are discussed in order to increase the time spent at moderate-to-vigorous physical activity in PE.