I'm highly allergic to crustaceans (shrimp, lobster, crab, crawfish), and just walking into a grocery store where shrimp was being cooked for sale at the fish counter has made me have to immediately leave and puke
Don't know or care whether it's the smell or something else, LOL - I get sick as a dog either way. My sister is also allergic and, in fact, just touching a salt shaker that someone else who's eating it has just handled makes her throat start closing up - I'm not quite that bad, except for when trying to dissect a crawfish in biology in 9th grade, before we knew for sure what was going on!
Both of us developed the allergies when we were jr. high school age (and at 53, I've not outgrown them, just developed some new ones), so we were already old enough to understand what we needed to watch out for, etc., but I think being sure the child is well educated and that teachers are informed and watchful is enough without having to police what every other child is allowed to bring into the classroom, etc.
And be sure there is an epi-pen in their backpack (or school office if necessary) and that they and teachers and caretakers know what to do if necessary. For severe allergies, Benadryl doesn't work nearly fast enough, although it can't hurt either.