I'd only add one thing: if a student is quitting band to pursue something else, wish them well. If they are quitting to do nothing, you really should talk to them in a diplomatic way about idle hands and the devil's workshop.
The kids who have quit for something else have almost always been fine. Those who quit without something to keep them occupied really have become some of the sketchier kids in school.
It's sad but true: band not only teaches an instrument, but it also keeps kids busy and when they lose that, they often don't replace it with anything that's going to be positive.