It depends on the nature of the misbehaviour. If it's something minor, and the child feels remorseful or embarrassed about it, then I think it's just best to discuss the misbehaviour and ways of avoiding this situation in future (e.g. How do you think it made X feel when you did that? What do you think you should have done instead? etc).
I don't think the school is wrong. It's for getting into fights.
In fact, I did say no pack of stickers, which we usually get on Wednesdays before swimming, and we've had a long chat about it, but DH thinks we ought to be doing something "bigger", but I don't really know what's appropriate.