It is unreasonable to expect a school to ban nuts because a small minority have an adverse reaction and an even smaller minority have a potentially fatal reaction. If you go down that route, then all sorts of things will get banned to point where practically nothing is permitted.
If your child has an adverse reaction to nuts, it is your responsibility as a parent to take steps to ensure that he/she does not suffer by both ensuring the school is properly informed and more importantly ensuring your child is educated to deal with the issue.
You cannot protect your child forever from situations where they may come into contact with nuts, so you have to ensure they know how to manage it and the sooner they learn the better.
Your are exhibiting a strategy seen all too often these days of risk avoidance rather than risk management. The reason behind it, all to often is to do things to "cover your back" rather than because they are reasonable things to do in their own right.