All good points, and I agree it's a "tell" not "ask" situation.
I think a pre-interview is a great idea, and a way to head of obvious issues/problems, but not all folks show thier true colours up front and....life happens.
Sometimes the reason someone is out of control is a life change situation (divorce, new habit, etc.). These are items that you might be able to discuss/help with, but usually the best option is to refer them to professional assistance/counseling.
But that brings up another interesting question...if a student poses a danger, and the best option is to tell them to leave (see, I learn
), do you give them an option to come back?
I realize this is probably a case by case situation, but if you WERE to discuss that they need to take some time to deal with a situation or reasses thier needs/situation, isntead of simply "leave and don't come back"....do you offer them a chance to come back and under what circumstances?