I don't know about government schools in the US, but at my kids' little parochial schools, each had a no junk food policy. I pondered one's right to eat junk food for all of about three seconds then moved on to more pressing life issues. But the right to slam one's lunch box shut and say "don't look at me teach!" -with teeth full of cheese biscuit - well, I couldn't have dreamed that one up. Even cops, where searches are otherwise not sanctioned, can say "but it was in plain sight in the back seat of the car" or wherever.