give them grace. maybe they are coming to reinforce their acceptance of you as the "new" guy. Maybe they are trying to show support even though they are not there any more. Don't assume. When I've left, I usually shut the door emotionally on the old school and buy into the new gig. The only time I've gone back is when asked by the new director or students. I did have some students ask me to "walk" with them on senior night and I said no. That was not in "good form".