Kids spend a lot of time at school, so there's been a lot of emphasis put on offering healthier food there, and more time to exercise.
But school may not be as much to blame as we thought.
Researchers looked at the body mass index patterns for nearly 20,000 kids, and found weight gain tends to happen during summer months.
In addition, the BMI's of obese kids tended to stay the same, or drop a little bit, during the school year.
Study leaders say more work needs to be done on improving factors outside school, like reducing screen time and increasing physical activity.
Study: For kids, more weight gains during summer