Julie,
I know exactly what you mean. It used to drive me crazy. Although, I cannot say I have never done that or I would be a hypocrit, but there is a time and a place for it, and it's not on other people's class time.
Maybe you should just say something to the instructor and they could say something subtle to the class or something.
That kind of thing used to especially make me mad when we were needing to get out of class but had to finish up first.