You cannot control the things that other people say or the way that they act, but you can control those things in yourself.
If they bring it up again, I would say something along the lines of, "I can't help the way I was born, but you can help the way you're acting right now." You don't have to be mean about it, but standing up for yourself will show them that they can't just push you around. Don't deny that you have autism; it will only make things worse, because then they'll find out that you're lying and that will only make it worse. You have nothing to hide! Autism isn't something that you chose for yourself, and you shouldn't have to hide parts of yourself just because other people are dumb.
If they still bother you, try talking to your teacher after class, or telling another adult that you trust what's going on and how it makes you feel; a parent, a principal, a counselor, another teacher, all of these people will know the best way to handle these kinds of situations.
Above all else, keep your head up! Don't let a few insecure girls ruin things for you, and don't be afraid to ask for help when you need it either! There are some things that we just can't handle by ourselves.