Just put a note in her lunch box next time. And EACH time. That it is soy butter.
From a school's stance.. they NEED to be vigilant about their no peanut policy, because if anything should happen to another child, they will be liable.
A school, cannot assume that it is or is not "peanut" butter. They have to surmise, that it "can" be. So it cannot be taken out.
Some peanut allergic kids, even if it is smelled, can go into a reaction and need an EPI pen, for example. Or, even if they touch a surface that another kid touched who had peanut residue on their hands... that allergic child can go quickly, into a bad allergic reaction. Some even get swelling of the throat etc.
So- it does not matter, if there are kids in her class with or without a peanut allergy. Because, any peanut reside or smells etc., can trigger a reaction in anyone who is allergic to it, ANYWHERE in the school. If there is peanut residue, around. Anywhere. And as you know, kids touch so many things with their hands, doorknobs, tables, etc. Thus, peanut residue, can spread. That way.
AND, this is a school wide policy.
The Teacher, even if you told her before it was soy butter, she cannot assume, everyday after that, that what is in your child's lunch bag, is soy butter.
They are not saying it is your child's 'fault.'