Background and objectives: Active commuting to school (ACS) is a promising strategy to increase the daily physical activity (PA) in youths. However, more studies are required to objectively quantify the mode of commuting to school, as well as the health impact of this behavior. Thus, the aims of this study were: (1) to objectively determine the mode of commuting to school using GPS; (2) to quantify the sedentary time, PA levels, energy expenditure, and the steps derived from each mode of commuting; and (3) to analyze the associations between ACS trips and sedentary time, PA, energy expenditure, and steps. Participants and Methods: A total of 180 trips to school were detected, which corresponded to 18 adolescents (12 girls, mean age = 15 ± 0.0 years old). Mode of commuting to school was detected using a novel method merging GPS data in the Personal Activity Location Measurement System (PALMS), whereas sedentary time, PA levels, energy expenditure, and steps were objectively evaluated through accelerometry. Logistic regressions were used to analyze the associations of these variables with walking trips. Results: A total of 115 trips were recorded. Most trips were performed by walk (49.5%), followed by vehicle (39.1%) and mixed transport (11.3%). In the active school trips, youths were less likely to spend minutes in sedentary behaviors (OR: 0.481, p = 0.038), a higher increase on Metabolic-Equivalent of Task (METs) (OR: 5.497, p = 0.013), and greater steps (OR: 1.004, p = 0.029) than in the passive school trips (both active and passive modes were objectively measured). Conclusions: ACS (mainly walking) contribute to higher METs and steps in adolescents. GPS could be an appropriate method to objectively evaluate the PA variables related to the ACS trips.