If it is specified by the school with the agreement of the parents that children should be discouraged from bringing unhealthy foods to school, then it should be policed as required.
I can also see the school of thought that says what's wrong with cheese biscuits and chocolate cake.
It really is a no win situation, kids do need to eat healthier food, and schools do need to be able to control the children in their care.
To my mind it's the fact that teacher's hands are tied by so many human rights, and do gooders rules and regulations, that they have no means of sanctioning these children. This is what is at the root of a lot of these things, children now feel they have more rights than their parents and teachers. No wonder they grow up expecting the world to dance to their tune.
G.