Do you think it's impolite or improper for a teacher to call his or her students 'guys'?

I feel that "boys and girls" and "children" are condescending terms. I like "guys" but sometimes the girls complain that they're not guys and I have to explain the generic nature of the word. "People" is best. Unless the cl is full of slacking monkey s, in which case the phrase "you slacking monkey s" works well.