Fair point about diet contributing more to obesity than exercise. And I am 100% on board with making school lunches healtier, and teaching kids about the importance of nutrition.
But exercise is also important for managing weight [1], and preventing/managing chronic disease [2]. And only about a quarter of adults in the US meet the HHS' activity guidelines [3].
What about building schools closer to population centers and provide pedestrian and bike infrastructure to it from nearby residential areas, such that kids could safely walk or bike to and from school?