Shemiras… The parents inform the school of their childs allergies. The school is obligated to keep the child safe in school. The school on its own determines the easiest way (for them) to keep the child safe is to ban peanuts.
If I was a school administrator, faced with a choice between dealing with a potential life threatening situation or a kid who only eats PB sandwiches, i’d opt for the latter. finicky eaters mature, dead kids dont.