The finding comes from a new study from Temple University in Philadelphia.
Researchers assigned more than 4,600 students from 42 schools to a diet and exercise program. They were offered healthier food choices in the cafeteria, in vending machines, and during class events. They also had longer, more intense periods of physical activity.
The researchers found that children who were already obese at the start of the program lost a significant amount of weight, compared with other children.
The overall numbers of overweight and obese children fell by 4 percent over three years whether the schools had such initiatives or not.
The findings are published online in the New England Journal of Medicine.