Oh my Sheena.
Reading how that principal treated you *really* P's me off beyond belief! BTW, having an older mother is a good thing - my mom was 43 when she had me.
My DH and I both said we see a lawsuit in the future. Banned from school property forever? I think not. I doubt that's even legal. You have DSS on your side, you've seen doctors, you even have a letter regarding your MS. Do not allow this principal to treat you this way. As a matter of fact, if it were me, I would take it to the media - they love stories like yours. I'd hate to think this principal would be allowed to continue to treat others with a legitimate illness this way.
Kitty had a great suggestion about home grocery deliveries. I sure wished they'd offer it where I live. My DH also has a disability and it's often difficult for us to go to the grocery store.
Please take care of yourself and let us know how you're doing.