The mother acknowledges that and does not believe what her son did was a violation of the agreement. So unless you can produce a copy of the agreement that states what her son did was a violation, then you are the dunce for jumping to conclusions.
We all can be guilty of that. My larger point is that if the mother and son signed an agreement forswearing whatever it defines as “disruptive” behavior - then the son commits an act of what the school believes to be “disruptive” - the mother’s actions in trying to override said agreement sends the wrong message to the son. In my view, that’s a big problem in our society - the lack of accountability.