Respect

In Germany college students wait to leave until the teacher is completely done talking. Then they knock on the desk for a second or two, telling the instructor that he did a good job lecturing, and then they get up and leave. All in all, it's a very respectful process.

In the U.S. college students start getting up as soon as the clock says it's time to go, some students get up regardless of the fact that the teacher is still talking. As soon as he pauses briefly, a good portion of the students will stand up. If he starts to talk again, they'll just stand there until he's done and some of them will talk still. There's no knocking or anything. All in all, it doesn't seem like such a respectful process.