What should we do? (bullying problem)?

[Katelyn]

My best friend is being harassed on Twitter... She's being called a slut, nasty, ugly, and someone even said she should stop breathing. She was forced to delete her Twitter because of all of it. She is very upset about it and was even crying during school. While walking through the halls, someone yelled slut! And hey theres the whore! As her best friend I am very concerned and fed up with everybody's rudeness. I have been sticking up for her through everything, and I have also been harassed because of it. We are thinking of going to the principal, but we are not quite sure what to do. Any suggestions? Thank you!