I lay down ground rules in my classes; anyone may disagree with me on matters of opinion, but not on matters of fact. I teach a difficult subject because it is so ephemeral - media studies. Here's an example: I saw James Cameron's Avatar, and thought it was...OK. Many of my students seem to consider it a masterpiece. It's just not fruitful to say that they're "wrong" and I'm "right." It's truly a matter of opinion. However, what is trickier is that I'll make a point about something like copyright, an area in which I've published, and some student will disagree. If it's a fact, I'll gently correct. If my interpretation of the fact is, in my view, a bit more en pointe, then I'll say that. I truly believe, though, that, no matter how much they complain, students respect you if you take a stand.