Contrary to the ridiculously self-indulgent and superficial school lunch reforms called for by certain celebrity chefs, the greatest harm to children's health comes from the forced inactivity which has greatly increased over the last twenty years. This is important, because children are, by nature, active. Inactivity is trained behavior.
It's been estimated that some 40% of schools have eliminated recess in the United States. From my own experience in our local school district, although recess technically remained, it had been reduced to about ten minutes, with no running or shouting allowed, by the late 1990s. I suspect our local district is not alone.
Inside the classroom, modern school designs often include no windows, closing off even a view of the playground. The rooms are festooned with video screens and electronic instructional materials - exactly the sort of environment any good parent would try to avoid creating at home. All-day kindergarten continues to expand, penning otherwise active children into these cushioned cells beginning at ever earlier ages. Extensive homework is assigned beginning in very early grades, ensuring that free time after school will be occupied with even more indoor sitting.
The public schools are notoriously unable to teach basic academics, but they are succeeding in one area: teaching kids to sit around instead of playing. You want them to slim down? Let them be kids.