I usually believed that when a rebbie threw a kid out of class, (my kid or not) it was for a valid reason. But I have come to learn over the years, that rebbeim are human beings who also make mistakes sometimes, and it is possible for a student who is completely innocent, to be rebuked for no valid reason. It happened to my son, I think I related the story here once,and the student who was actually guilty immediately came forward and said my son had done nothing wrong, but the rebbie kicked him out anyway.
He called us in for a meeting at school, and he was very embarrassed in the end, when he finally came to realize his gross error. We were not looking to bust his chops, but we did express our dismay that he had not been dan l’kaf z’chus to at least listen to my son’s defense. He acknowledged his error and to his credit apologized to my son for the embarrassment caused to him.