There wasn't a lot we could do at the time, even with knowing it was going on due to the parent. Long story short, while it was bad for us within the initial bit, it probably worked out better for us in the end. The year this kid did that to us we went and earned the first sweepstakes the junior high had ever had.
I will say the part that really hurt was that it was the same time we had a terrible coach. He would bad mouth band all of the time, and actively worked to get kids to quit. With these kids involved during his time we lost most of the football players that did band too. It's really taken until this past year to really get back to a decent place there (about eight years).
Like I said, it sucked at the time. With your smaller size it'll be even worse... With us being out, it's not exactly easy to fix your problems. You could go ahead and start calling parents and see what you can do with them. If they invested in instruments you may have a little bit better success. Maybe your admin can help you too?
Being honest though, all of us are probably going to have retention issues going into this new year. If anyone figures out a magical solution please let me know!
Oh, and those kids that did that later got to see the band go to state marching contest... and let's just say the team didn't. Karma has a way of working things out!