I've been in your shoes. I threw 1 girl down the stairs, she shut up after that. I was 15.
Option 1. Ignore them. They are a bunch of dumb b*ches who just want attention. Be better than them by pulling really good grades and making new friends.
Option 2. Stand up in class and say everything you've heard in front of everyone. "So, I like to make an announcement. Rachael, Beth, and Adriana were mentioning to Kate, John, Austin, Tommy, Carlos, and Sharice that I am anorexic and that I have been throwing up in the bathroom. I just wanted to clear the air that I do not have an eating disorder, and if anyone cares so much they can reference my personal doctor. Thank you." and sit down.
Why? because it's something they will totally not expect.
or you do the last thing, throw 1 down the stairs, serve juvi at 15 like I did, get in huge trouble by your parents....but walk back into school knowing nobody is gonna f*** with you.