Looks like they all beat me to it, get a rolling backpack. There's no excuse for knowingly aggravating your chronic pain condition, you will regret it.
Talk to your school about special accommodations for students with disabilities. If you don't stand up for yourself, everyone will walk all over you, speaking as a chronic patient, you need to be on your team and call the shots. Don't unnecessarily put yourself through pain. It took me a while to realize all this myself. I kept feeling hopeless and wondering why no one was helping me, or if life would ever get easier.
I'm not saying that you're like me or anything, but hopefully you'll see what I'm getting at, that you are the only person who knows what you're going through and that you must act accordingly, others won't always know what to do about it. Your parents are on your side but that doesn't mean that they know what you're going through. If you're having problems with pain, and the backpack makes it worse, but you just keep wearing it anyways, they're going to think that it's fine.
Good luck bro, I really hope that you find a way to cope, I'll keep you in my thoughts.