I think you should talk to someone higher up then your teacher. Does your school have a code of conduct? I don't think you can be suspended for saying a word which is not considered offensive, nor do I think you can be punished by your teacher for stating your opinion in class, espeically when you we're asked for it. If you have a student handbook you can show it to your teacher and your school administrators if you have to, in order to prove you are not breaking any rules by saying the word sexist.