My oldest daughter was severly allergic to eggs, dairy, citrus and nuts. And yes, it was frightening to send her off to the care of others who weren't totally tuned in. But she's 24 now, and she made it through fine. People really do care but you have to be extra vigilent. Here's my advice:
(1) start training your kid to ASK ASK ASK "are there nuts in this? I can't eat nuts." He needs to say that to EVERYTHING.
(2) meet with ALL the teachers, aids, school nurse, principal, EVERYONE, even the manager of the school cafeteria. Make sure everyone knows just how serious a reaction could be. Leave them with a written list of allergies and instructions.
(3) Give an epi-pen or benedryl (whatever your doctor suggests) to the school nurse and the teacher.
(4) If your school asks parents to provide snacks, make sure you give your school plenty of snack food to give your son if the brought snack contains nuts.
(5) Before a school event or party, remind the teacher that your son is allergic.
In short, you have to be vigilent until you are POSITIVE that your child's teacher is on top of it, and until your son is old enough to monitor this himself.