Learning to sit and focus for those 8 hours, gaining principles of education is it's own value.  This is separate completely from the eating habits that are causing kids to become fat fucks.    And it's so easy to fix, bagged lunch, complete control over dietary intake.  Don't let your kids eat breakfast at the school...(And btw, what the fuck is up with that anyway)  Kids aren't going to be trading fruit cups and carrot sticks for shit from the vending machines.  After school activities and play handles the activity levels.  This is not new, there is years and years of kids attending school gaining an education, and not becoming mini-moons.