It all has to do with respect. If you respect me as the instructor, then I wil respect you as the student. This means that when I am in front of the class lecturing that you are not being disruptive or disrespectful. And, when you are speaking or representing your point of view, that I am respectful of you. I feel that it is very rude when another student is having a private conversation with their friend while another student is trying to answer a question or the instructor is speaking in class. But, in addition, I feel that you have to establish those ground rules in the beginnning before the behaviors get to that point.
