About a week ago, a kid in my Torts class got taken to task for dropping that little bomb. Our prof told us a story about how he was called on virtually every day by his Contracts prof in law school (30-40 years ago). One day he honestly had no idea about the answer to the question. He answered "I don't know." The prof moved on, and never called on him again. This sounded great at first, but then my prof realized that "I don't know" meant you didn't have anything good to say and you had given up. So his professor found a new teacher's pet.Bottom line, even if your teacher hates "weaseling" (another on of my prof's pet phrases), it's always better to BS than give up.
If half the class BS'ed all day I wouldn't learn anything and would easily get confused and distracted.