Professor So-and-So?

It depends on the context, sort of. Generally I prefer calling them Mr./Mrs./Professor whatever, since in my mind that's the appropiate student/teacher relationship. If things were to progress to the point where we might be casual acquaintances, then I would probably feel more comfortable calling them by their first name.

Really though, it's mostly what the teacher tells us to call them. I had one Statistics teacher who insisted everyone call him "Frank" (because he was) and that's what I ended up doing. Though, I usually sidestepped the problem entirely by avoiding saying his name. <_<

Jam it back in, in the dark.