Well, this may not be the most mature thing to do (although those people aren't being mature either), but I would say something back when someone makes a comment. When someone says "retard" look them in the eye and say "actually, I'm probably smarter than you"; "ewww look at her" say "that's funny, I thought the same thing when I looked at you" or whatever. The usual advice is to just ignore bullies, but sometimes it doesn't work...at least this way you might feel a little better

.
Thankfully I haven't delt with this too much, except once when I was in high school... two boys and a girl were walking down the hall a little behind me. The girl said, "Aww I feel so bad for
those people (in a normally loud voice, like I was deaf or too dumb to hear

) That in itself didn't bother me, because most people don't realize that we're normal and the disability is just a part of everyday life. Anyway, the boys she was with were the delinquent-looking type, if you know what I mean, and the one said (again in a normally loud voice) as they walked past me "I should touch her and make her go crashing into the lockers" or something like that, laughing and imitating pushing me. Well I wish he would've tried to push me because I'm stronger than I look and probably wouldn't've fallen over, and then I could've laughed in his face, haha. But by this time they were ahead of me so I said "HEY," they turned around, and I said to the girl "I feel bad for you too if you're friends with him" snickering. The look on their faces was GREAT.
Anyway just try to keep your head up, we can all sympathize with you. You should feel sorry for those people because they are ignorant, insecure, or both.