I've learned that, the vast majority of the time, trying to talk those kids back into band doesn't pay out. If they're wanting to quit, their connection to band was tenuous already. Convincing them to stay in will typically give you a kid who is barely committed to being there, and in my experience, they tend to sit towards the bottom of the section anyway and have attendance problems. I have realized that I would much rather have a band of 50 committed kids with great attitudes than a band of 70 with a bunch of flakes.
My advice would be to focus on the kids who are staying, love band, and want to be there rather than the ones who are leaving. Give those kids your energy and excitement, and you'll have a group of kids who makes you excited to come to work each day.