Please let me take the opposite side...do you really want to eat something cooked in a kitchen that might have maggots in the sink, week old blood from thawed meat on the bottom of the plate the cupcakes are sitting on, etc...you really don't know what kind of conditions those treats are cooked in. Food poisoning of a whole class isn't funny. How do you know the people cooking them don't have drugs that accidentally might get mixed in.
This is the main reason that more and more facilities are making only store bought items allowed, with the seal intact. For kids safety. I agree with the concept. Only health inspected kitchens. The school has to have their kitchens inspected, so should the food the kids eat for parties.
So, go to familyfun.com, or parentcenter.com, or some recipe websites and look up healthy snacks in the search engine. You can get some very healthy ideas that can spark your imagination and give you inspiration as to what to buy. The kitchen at your school may be able to give you ideas too.