School uniforms could be restricting young people from being active, particularly primary school-aged girls, according to a new study. The study used data about the physical activity of more than a million five-to-17-year-olds in 135 countries. In countries where a majority of schools require students to wear uniforms, fewer young people are meeting the World Health Organization's recommendations for physical activity (60 minutes per day). Fewer girls are meeting the guidelines than boys -- with a standard gap of 7.6 percentage points between boys and girls.