Document, document, document.....and then document some more.
Keep track of any and all phone calls and/or correspondence sent or received. Anything you send...send it certified.
I'm not sure what the laws are in your state but if it's allowed I'd take a small tape recorder to any meeting held with that choir director or the school principal. Geez.....am I sounding paranoid or what??

In this day and age of sue happy people you just can't protect yourself too much. Especially when it comes to your child....I'd rather be overly cautious than too laid back.
It's sad that it has come to this. Kids (and adults) can be so cruel. I just do not understand the mentality behind it.