Ali,
When your classmates ask "what is up with you?" and so on, I hope that in reality they are just expressing a natural curiosity and, in their own way, have some concern for you. Do not forget that as children grow and mature, they are still learning about proper social skills and how to express their feelings. Someone who said that might have been uncomfortable and not sure how to speak to you. Same thing with your so-called "friend" who said you could not sit next to her. She may have been unsure how to interact with you.
As I am sure you have noticed, at age 13 young people have all different levels of maturity and wisdom. You seem to be very mature for your age. Others may take a while to "catch up." This is common. If a classmate acts rude to you, remember that they are faced with a scary thing here as well -- a person who cannot run around and play the same way they do. There may also be some subconscious thought that they, too could become "disabled" like you. This may cause people to say or do inappropriate things.
I hope that, as classmates get used to you being there, some of them will figure out that you really are no different on the inside as they are. It may take several weeks, but I believe it will happen. You may even find that your "old friends" are not the ones to accept you, but a whole different bunch of kids. You will probably find some good friends for life.
Hang in there,
Mike