What should I expect in a college philosophy class?

Some professors are indeed like this, but it's pretty rare. To be honest, I think this rumor comes from students not understanding their professors' objections to their views. Some professors want all of their students to be atheists, but most just want them to think clearly. Unfortunately, though, when a student writes a paper claiming to prove the existence of God, and the teacher criticizes the argument, students sometimes take this to mean that the teacher wants everyone to be an atheist, and is biased, and wants everything done "his or her way."

As for the preachiness, most philosophers have arguments. In (most) philosophy classes, that's all we care about. Find the argument, make sure the argument is what you think it is, and then feel free to criticize. There's nothing wrong with this, so long as it's done carefully, so long as your objections make logical sense. One caveat, though, with historical texts. The emphasis in history-focused classes is more often to figure out exactly what the author is saying. Evaluating the argument is important too, but a large part of those classes consists in carefully reading the text to figure out what the author's getting at. And this is important, even if a lot of it's wrong: philosophy grew out of these texts, and they continue to inspire original thought in nearly every field of philosophy.

I just completed my undergraduate in philosophy, and these problems you outline were never a large concern. Most of my professors were upfront with any relevant bias they had, such as being a staunch Kantian deontologist in an ethics class, and then taught the material quite fairly and charitably; sometimes, a joke would be said by a professor coming from their bias, such as calling utilitarians "nutty," but it was said in good humour and often self-deprecatingly to boot: "Bentham's point here is just nutty, but what do I know, I support the view of a guy (Kant) that no one can even agree that we understand completely." What little preachiness that came from professors was similarly humorous and self-deprecating. In my experience, the issue is usually students: some students will obstinately hold to certain philosophical tenets and try to preach about them every class.

Many of my introductory classes were taught in what I was told is "seminar style," which was that we were expected to have read the assigned reading. In the next class, the teacher would reiterated, and for some including myself taught for the first time, the central concepts of the text that we were assigned.

Then we'd raise our concerns, questions, or even challenges to the text. The teacher would then either respond to the question directly, develop the question and then address it, or deflect to someone in the class who might have their own counter-response.

Basically, as far as my experience to the introductory philosophy courses, the best classes became a game of pong in which some students adventured responses and others remained in respectable contemplative silence but there was a general sense of a class progression toward the material.

Pretty much the same thing to expect in any other university class, which is to say it's very hard to know, absent further information, because there is massive variation from professor to professor, institution to institution, class to class, year to year, etc.

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If you are unlucky enough to find a professor with a marked point of view, either adopt his view or leave. There is no middle ground. Most philosophy professors, however, are not like this.

I've read about professors teaching one sided, for instance they want everyone in their class to believe what they believe and that only. Is that something that's notorious in this field?

I've only seen this in weird strawman depictions of philosophy classes (e.g. "God Is Not Dead"). I've never experienced it. One of my favorite things about philosophy professors is their ability to convince you of something they themselves don't agree with. They're acting as a proxy for the philosopher they're teaching at the time.