I think schools should stick to teaching tbh. Any appalling lunches so eak to the individual parents.
No nuts no sweets and no fizzy are probably standard sensible rules.
Besides that the usual suggested alternatives are often as bad as or worse than the banned items. As if a penguin is that much worse than a yoyo bear.
Ban a cake but serve a flap Jack with school meals? Yeah whatever 
Warm water stood all day in a classroom wouldn't appeal to anyone. Squash would at least make it drinkable.
Either provide a fridge or put up with it.