"It's always better to BS than give up."
It really just depends on the teacher and his or her personality.
A few weeks ago in Legal Analysis, a guy was called on who was either not prepared or just hadn't read very closely. Either way, he attempted to B.S. his way through the material. The prof just went to another person and didn't come back to the guy.
Two days later, in the next L.A. class, the prof came in and says "Here's the deal. I hate bullshitters. If any of you attempt to waste my time by bullshitting me again, I will drop you a letter grade. Not only that, you can expect to be called on for the next three class periods, and at that point you had BETTER be prepared." He did go on to say that there was a difference between being obviously unprepared, and not knowing the answer. He said it was better to admit that you don't know, rather than trying to fake it. So I guess it just depends on the teacher.
Coincidentally, the guy who tried to bull him in the first place was called on for the next two class periods. I haven't seen that guy around in two weeks. I think he dropped.