Very brave of you!! I think the evaluation may depend on why a student is quitting and may not reflect upon you as a teacher. In other words, say the student is quitting because they never practice (nothing to do with you) or the parents got mad because you charged them a late fee for a late payment (just an example!!). You then give them an evaluation form and you may get very different answers as opposed to students who are quitting because they are leaving for college!

I think if it were me, I would give them the evaluation form before they quit, while you are still teaching them.