I’m gonna get absolutley burned for this, not suggesting what you saw was in any way acceptable behaviour but….
….my other half is a primary school teacher and has to deal with a lot of shit on a daily basis. The amount of times that the parents absolutely disbelieve the school about the behaviour of their little darlings far far outwieghs the times that they take any kind of action or responsibility. Furthermore the school never singles out kids just because they don’t like them or their parents.
Like I said, no excusing the behaviour that you have witnessed and I’d be inclined to take it further, but have you really asked yourself whether your kids have behavioural issues? I’ve never ever been called in for my kids, and they can be right shits at home sometimes.