If people are adopting a "grazing" style of eating, perhaps that's something school lunches should adapt to instead of resisting.
The societal costs are too high -both direct and indirect. It is not even remotely responsible behavior to say because kids like eating junk food we should just knuckle under and give it to them. They might also like staying up to midnight, drinking booze at 12, and not studying any math - but we as adults don't let them just do whatever they want.

And if people don't like the school food they can always bring their own. Easy choice. No one is forcing them to eat the school lunch. No one.