I knew I would post a response to this as soon as I read the title. So here goes, I personally think the saggy pants are annoying and show a level of slovenliness that I am not comfortable with. BUT this is a school, where young people go to express themselves, by fashion and personality (oh yeah and learn stuff too). My point is that if it is actually a safety issue, and you are actually concerned, than by all means determine what is and is not appropriate and police it. If it is a personal irritation (as it is for me) I think as a teacher you have right/responsibility/whatever to allow students to dress whichever way they want. The responses to this thread have been advice on "justifying" your personal needs, because YOU don't like a dress habit.
My 2 ¢
Robert
The beaten path, is exclusively for beaten men.