[AlkalineTears]

Get a voice recorder, if you have an mp3 player or ipod or something similar and record them making fun of you.

Go to a psychiatrist, or your doctor and tell them what they are doing. Because that is abuse, and you can use it as evidence.
I would make reports to people like the kids-help line, or even the police for abuse. Some might say its a bit far to do, but their parents need to be taught how to educate their kids to not make insults to people who have disabilities.

I understand thats a lot to do at 13, but it will benefit you in the end and make you a stronger person knowing you took action.

You can find people you can trust, it will take time but it will come eventually. I was a victim of abuse like you but for different reasons. If only I knew that I could take legal action against abusers even at such a young age I would of been in a safer environment and I wouldn't of had to put up with nonsense from people.