<p>It's really most courteous to arrive before class starts to talk to the prof and ask if it's OK to sit in, and then stay for the entire class.</p>
<p>interesteddad: Well, I've certainly seen parents and prospective students sit in on a large lecture class together at the back row, and the prospective student (I was sitting nearby) didn't seem to mind at all. Let's put it this way: Parents should ask their children if they want the parent to be with them or not. If the spec wants the parent to attend, then fine. Ultimately it's between them two. None of the current students really cares what they decide.</p>
<p>I agree with dadx3, don't pop in halfway in between, because it might disrupt the professor and the students. I'd say it's better to stay for the whole lecture, or don't go at all.</p>