The objective of this study was to investigate the differences in the volume and intensity of physical activity (PA) between girls and boys with different levels of academic achievement (AA) throughout individual segments of a school day. In all, 136 girls and 76 boys aged 16-19 participated in the study. Girls and boys in individual classes were dividedinto groups based on AA as determined by their most recently issued school grades. We used ActiTrainer accelerometers to measure the volume and intensity of PA and heart rates. Girls with worse AA engaged in more PA than girls with better AA during the time periods before school, after school, and throughout the day. No significant difference was observed among boys, regardless of their AA. Significant differences in PA, volume, and intensity were not confirmed by heart rate monitoring. The highest step count per hour of accelerometer monitoring was seen among boys with better AA (1467 steps•h-1) and girls with worse AA (1364 steps•h-1) during the time period before school. To promote a lifestyle that includes PA for adolescents, girls with better AA and boys with worse AA should be closely monitored during school and extracurricular programs.