This may also be a good point for some hands-on education of the principal, nurse, and possibly other staff.
If you can afford and have the time, I'd go buy a bunch of different candy canes -- some with warnings, some without, and some with claims of "X allergen free" . . . and also print out a copy of the food label laws. Have a brief meeting and education session. Let them try to decipher "safety" and define "harmless" . . . don't do it for them. Leave with the candy -- NONE stays there.
You can also do this with photos of ingredient lists from candy, if you don't want to buy. Make a powerpoint lesson and email it, cc'ing all who need to see it and get it.
I think there needs to be an eye-opening lesson that does NOT entail the injury or death of a child on their watch.
This isn't just about your child's limitations (or not) as to self-advocacy. This is about the adults' limitations as to understanding that they are NOT to feed your child or even OFFER food. You have to illustrate the "why".
Sorry you are having to deal with this!