To study the effect of a teachers' ban on supervising sports-related extracurricular physical activities (ECAs), levels of physical activity among 979 grade 7 students (mean age=12.7 [0.5] years at baseline) were compared during and after the ban in seven schools that fully implemented the ban, and three schools that did not implement the ban fully. On average, schools offered 18.0 (SD=5.1) ECAs during a no-ban school year. Students attending full implementation schools were significantly more likely than students in nonimplementation schools to be active after the ban ended (odds ratio for being active=1.89 [95% confidence interval: 1.39, 2.58]). They also increased the number of physical activities in which they participated (coefficient=4.04; SE=1.01). Ending a teachers' ban on sports-related ECAs was associated with increased involvement in physical activity among secondary school students.