The rare times I have addressed this with teachers I have said, my child said this happened, that can't possibly be the truth can it? Sometimes it is but, usually I am not getting all the story. Sometimes I have had to escalate things, sometimes the teacher remedied the issue. But I would always start at the lowest lever before jumping to administration. It is a lot easier to tell the administrator that the teacher is not being reasonable when you spoke to them about the situation then explain to the administrator what you think is going on, which may or may not be the whole story.