well, discipline is one of the biggest and most talked about subject when it comes to child rearing, and I've read several books from a variety of psychologists, researchers, doctors, and the list goes on. (i'm a book junkie, when something cathces my interest, i keep delving into it until i feel the subject is worn out)
first off, you asked him why he did it. he said he forgets to be nice and to listen BUT HE CAN'T HELP IT. that in and of itself is an invalid excuse. he places the blame on something else other than himself. he can help it, but he will have to work harder at it in the future.
work with him on "choices" when he "chooses" to act nicely, and "chooses" to listen, he will be happier with himself, and others will be happy around him as well. If he chooses to not act nicely, he will be responsible for it.
another thing to ask him, is if he thinks he is a nice boy. if he reponds with yes, then explain to him if he's already nice, then he must be choosing to be impolite.
now on the other hand, the teacher is not equipped to get positive results from students herself, if she is constantly throwing him out of class. make it a point to meet with this teacher, and tell her what you are doing, and let her know that you respect her as a teacher, that some kids need a different approach to achieve desired behavior, so if you are doing certain things, like using communication with him differently that you would appreciate it if she could follow the same guidlines as you are setting, and they are NOT huge changes. I would highly recommend reading "what did I just say" by Denis Donovon. everything i just told you would make more sense, after reading that book.
your child is most likely perfectly normal!!! he probably just hasn't learned how to listen/behave/follow directions in the "normal" method, which would be the majority of the kids, but everyone has different ways of getting to the same goal (listening, behaving, and following directions)
please, read the book. I think it's a great tool to be prepared with