More on-topic, a friend of my daughter has a tree nut allergy, apparently. I have no idea if it is "real" (meaning a challenge test, since I never asked). We were at a party last weekend, and her mom told the other mom about it, then since the girl knows me pretty well, she asked if I could be the grownup that Alexandra could ask if she had any questions. I said sure, then asked a few questions of mom so I'd know how to answer
Anyway, for this girl, peanuts are fine, just not tree nuts. As a result, her mom sorta pushes peanuts since it's a "nut" (not really) that her daughter CAN eat, whereas other allergy kids can't. So she revels in eating peanut stuff, and brings peanut treats to replace possible tree-nut treats. In this example (OP post), she'd be intentionally armed with a deadly weapon in school, lol. Course so would my daughter given her love of PBJ, or PB, banana, and honey sandwiches.
I know in my son's preschool class that we used to make an effort on snack day or party days to have something for allergy kids even when we know they keep special treats for them at school in case of parties, but the teachers ended up not using it because they could not be sure without the say-so of the right parents (and what if something was contaminated? Yikes!).
I think the responsibility should be 100% on the parents of the kid in question, plus the kid. Any additional effort on the part of the other kids is inappropriate, IMHO (and ultimately not something the kid with allergies should count on, anyway, it's a false sense of security).