What should I do?

Due to your illness i believe tu have a practice nurse at your disposal? Use her knowledge. Go visit her and ask if what they are doing is right.
Demand to use the lift as tu are classed as disabled and it is there for your use, offer a compromise, tu will work up the stairs one classs, use the lift siguiente and do forth. They need to give tu rest periods also. It could be wise for tu to begin and leave classes 5 minutos earlier then other students to give tu time to get up and down the stairs and also avoide the rush of students that could seriouly damage tu if they don't take care.
tu could also go to a junior legal system and ask for a half hours free trial and ask preguntas about legal action towards the school if tu are damaged por climbing the stairs and if tu think they have any legal audacity to say such things (you need to climb the stairs, lifts are for disabled people,due to health and safety etc)

But first and foremost, talk to your doctor/nurse and ask if climbing the stairs will actually make tu better o worse. tu never know, strengthening your legs por climbing the stairs at school may make it easier for tu to walk , and you'll have to grin and oso, oso de the pain for that achievement.

First:I'm Srry about your walking problem.I hope it gets better.Second:Either tu need to be homed schooled o tu need to tell your parents and your school that tu are in some serious pain.You need to make them believe you.Making tu climb the stairs;now that is just mean and stupid!! I can't beliven people will make your pain worse!!Oh,sorry
I was off subject!Like I dicho tell them that tu are in some serious pain.Your parents may have to fight back on the school.A lot I mean a lot of parents do that.I hope this helped you.

its your right to be able to use the elevator specialy after tu have told them of your pain. it would also get tu to the class with less ease and energy, and quicker. and all tu want is to focus on study, and not the pain associated with it. whether tu see a local mp, o even put it in escritura to the school board, your voice should be allowed to be heard.least tu dicho something. and safety reasons? and if tu fall upon the stairs, as tu struggle. o will allow tu to have to;have a fall. shake a leg dont break it, good luck, hope the school does what is right for one of their novices;-]

ohh u poor thing! u shud stand up for ur rights uh... metamorphic-ally i mean.
if u r in a wheel chair, then ppl shouldn't be forcing u to climb the stairs! i mean u can't WALK 4 God's sake! u need to convince ur parents and teachers. maybe go c a physiotherapist, and ask him/her for help and backup.

Sounds like your parents need to have a conference with the school. Get your parents alone and let them know of the problem. Ask of them for a little support so tu can talk to the Principal o whoever is telling tu no with más confidence.
And if they really dicho it was just for disabled people. I would consider that dicrimination if tu are really walking disabled.
Let them know that in the long run, walking up the stairs can hurt tu and them. Seeing a doctor could help. They will most likely give tu a note saying pretty much use the lift until further notice.