Well, I'd go for it.
Sad to hear that it's getting worse tho.
I recommedn you a) ask the doctor for some papers proving that you have a bad back, so that your teachers can just stfu. b) Ignore them. Ignoring people is a really smart thing to do in some cases. If they talk behind your back like that, then they're not realy friends. Sorry.
I can just tell for sure that I'd not let people go around talking behind my back, if those were lies, but I'd also ignore them, depending on the case.
As I said, even if there's a small chance of success, I'd go for it. (I don't want you dead though.
)
If you think it's hard to ignore them, just give them bullet proof proof!
They may still doubt it, but then, they are plain assholes (IMHO).