
A recent article in Newsweek Magazine states that the Cochrane Collaboration found that school-based exercise programs don't do much if anything to help adolescent obesity.
That's because school based lunches are rectangles full of carbohydrates. It's also because most kids breakfasts are primarily carbohydrates.

And what's for dinner?

Let's not even discuss desserts and snacks - please.
Sugar in all it's forms raise the hormone insulin. Insulins main job is to store body fat. Keeping insulin levels low inside kids bodies should be the target of schools, not forcing kids to run around a little more often.
Activity within the body is far more important for helping to stay lean and healthy than activity outside.