It is your responsibility to educate your child and those around him about his own allergy. By the time he is old enough to go to school, he ought to be old enough to understand, "Only eat what mommy packs you for lunch. Do not eat any candy offered by anyone. You could die."
And don't think I'm not sympathetic...my youngest child has Celiac disease and there are TONS of things she can't have. Wherever we go, I make sure she has her own food, her own treats, etc...and even though she's only 21 months old, she already understands, "You can't eat that even though (so and so, including your sister) can, because it will make you owie."
If we block schools from having peanuts, then you also need to stop them from having most fruit, some veggies, bread, crackers, cookies, milk...what are they going to be left with? This is how I look at it. There are allergies to almost everything. Just in my family, between us all, we can't have milk, wheat, barley, rye, oats, peas, nuts, seeds. Do you realize how much that eliminates? Fortunately, that's between four people.
I understand how frustrating it is. For example, at our church nursery (of which I am the director, so it was my say) I banned all food except potato chips from the infant nursery to protect my daughter. The toddler nursery has all kinds of crackers, cookies, etc...and when she's old enough to transfer, I simply won't be transferring her because I'm not going to take away food from 30 other children just for her sake. There are only 3 babies in the infant nursery, and they (until a point) aren't eating solids anyhow.
And then you get into what parents just don't want their children to have. Some parents are so set against high fructose corn syrup, for example...so there go your starburst and skittles.
I homeschool, by the way. I feel it's my responsibility to protect my children and my responsibility to make sure they still get the education and socialization (far overrated) that they need.
(And just another view...peanut butter is one of the few things my toddler can and will eat. Should I have her stop eating it because someone else can't? Fine. Then all the children also can't have bread, cake, cookies, muffins, crackers of any kind, any kind of sauce or condiment, any kind of cereal, any flavored water, and drink EXCEPT plain water. According to my child's diet, should I say that the whole school can only eat whole foods consisting of organic fruits and veggies, rice, and peanut butter? Because that makes up the bulk of MY child's diet.)
I hope that just helps you to understand the catch-22 of all this. Not that I don't sympathize...but like someone else said, where do you draw the line? As parents, it's OUR responsibility.
(I also see a few people going back and forth on the severity issue. Well, sure, my child won't die if she ingests gluten. But it can cause cancer. So yes, she could die as a direct result from consuming the foods she isn't supposed to. It also makes her intestines bleed...and that is an immediate result.)