I want to make one little tiny comment here. People need to understand the difference between being polite and using their brains. If you have a knee condition, don't sit in seiza for a long period of time. If you are in severe pain, you won`t be watching, listening, or learning. And THAT is what is seroiusly rude.
(of course, make sure your sensei knows about this...it'll stop the glares before they start)