It is impossible to ban all allergens, there are people allergic to every single food group, every medicine, every plant, somewhere. One may be allergic to tomatoes and one to lettuce. Do we ban tomatoes and lettuce from all school meals? No, we have parents educate their children and register them with the school so they can be provided a lunch that is safe for them. Do we tell all the students in the school they can't have lettuce and tomatoes no.
No one is trying to throw any kids under the bus here, but schools cannot control every food item brought in by kids, it's impossible. Parents send lunches they want their kids to eat; can we search every lunch? No, we can't, it's against the law. Do parents give their kids snacks and tell them to hide them in their backpack, pocket, whatever? Yes, they do, I've run into that so many times. We do not allow those energy drinks at school, especially an elementary school, but if the parent thinks it's ok for them to drink it they send it, tell them to hide it, and drink it outside at lunch. If we see it we take it away and call the parent and get told that they think it's ok and it doesn't matter if it's against the rules. So then their kids ends up eating in the classroom on detention for their parent breaking the rules.
There is no way for either side to win this argument, it's not going to happen. Live and let live and just be careful.
The one thing I ask of my parents is to stop telling their kids that "being on the grass will kill you!!!!!" (example!!) The kid has an allergy to grass and shouldn't get down and roll around in it during PE great, but don't scare these poor little kids so badly they cry during fire drills because they have to walk by the grass the parent has tried to terrify them into staying off of. It's like using the doctor to scare kids into behaving,,, "If you don't behave I'm going to take you to the doctor and he'll give you a shot!!" Then they can't understand why the kid gets hysterical in the doctor's office!