Even constraining to just allergies, Oral Allergy Syndrome is very common. It's one of those things that people around you probably have and you never know it. Personally, I developed an allergy to carrots and celery when I was around 10. When I eat those vegetables raw, something resembling hives breaks out inside my mouth. It isn't actually dangerous, so I never mention it, and just happen to avoid certain raw veggies.

And that's an important distinction. Many people have food allergies, but only a small percentage of them get anaphylactic  reactions. And of those, a small percentage are sensitive enough to react to trace amounts. Really, these cases are few enough that schools should be investing resources to protect such kids on an individual basis (like the aforementioned caretakers scrubbing down rooms). Blanket bans that constrain the diets of millions of kids seem hard to justify when you break it down that way.