Background: Physical activity (PA) is one of the most important health behaviors that may be modified by each individual. To foster PA in adolescents, a school-based intervention was evaluated. Methods: A cluster-randomized controlled trial with preassessment in 2014 and follow-up assessment in 2015 included 29 schools with 1020 students (47.6% girls, mean age = 13.69 years). Intervention students received pedometers and monitored their steps for 12 weeks. Classes with the most steps were awarded. Primary outcomes included moderate-to-vigorous PA, out-of-school sports activities, active transport assessed through questionnaires, as well as cardiorespiratory fitness measured using the 20-m shuttle-run test and anthropometric data (weight, height, body fat, and waist circumference) assessed by study staff. Results: Significant interaction terms between group and time were found for all 3 indicators of PA; intervention students showed a higher increase of PA than control students. The same pattern was shown for cardiorespiratory fitness, but the effect missed significance. A more favorable development for the intervention students was shown for body fat and waist-to-height ratio, while there was no effect on body mass index percentile. Conclusions: An easy way to administer school-based PA program may enhance students' leisure-time PA even 1 year after the intervention has ended.