Question: Is my teacher allowed to tell us the Iraq war is wrong, preach his left wing politics, ridicule President Bush, etc. in class?
Answer: You ask an excellent question, and the answer is not a simple "yes" or "no." If you attend a private school, the teacher can likely say whatever she/he wishes concerning the issues you mentioned, as long as the teacher was authorized to do so by the school. If you want to know whether the teacher had authorization, perhaps you can ask the teacher directly or arrange a discussion with a school administrator.
If you go to a public school, then the issue is more complicated. The First Amendment to the United States Constitution allows all citizens, including your teacher, freedom of speech. But of course, your teacher, while in the classroom, is not an ordinary citizen. She/he is very different from, for example, a protester at a rally denouncing or supporting the war in Iraq. She/he is different precisely because she/he is a teacher, and her/his job is to educate her/his students.
But what constitutes education? Is a teacher "educating" her/his students when she/he tells them, as you put it, that the "Iraq war is wrong?" Maybe. The answer lies in the context that those statements were made. Certainly, part of any teacher's job is to stimulate class discussions, and one way to do that is to state a controversial idea or opinion in order to provoke response from the students. For example, a teacher who begins a discussion by stating, "I think the Iraq war is wrong," may be doing so as a way of encouraging you to think about the war and develop your own views about it. On the other hand, a teacher who tries to pass off her/his opinion as though it was not her/his opinion, but an actual fact, seems like she/he may not be doing her/his duty to present an issue in a fair and balanced manner.
Ultimately, teachers at public schools are city or county employees, and as such, the city or county sets rules about how classes can be taught. The teacher must follow those rules.