I am being bullied, what do I do?

[Mac]

I am 15 years old and I am a freshman in high school. I have gone to private school my whole life and now for high school I am at a public school. I didn't really know anybody when I first went to the school but I made some good friends that I like and who like me too. However, there is a group at my school who everybody knows. You might call them "the popular group" or the "cool kids" or whatever. Anyway. They have been saying mean stuff about me behind my back and I am hearing about it from my friend. They have called me "weird", "prude", "overachiever". They have made fun of me for not doing drugs or drinking alcohol by saying "oh i bet she gets high off caffeine" and when I followed them on twitter they said "ewwww, im not following her back!" They have said all of this about me but have never met me or talked to me. I have only heard these things through my friends. These comments really hurt me and have caused me to be self concious, they have caused me to be less confident, and they have even caused me to self harm ( i know you think i am over reacting but i have a lot of other things going on in my life right now so they are not the only reason for my self harm).

Is this considered bullying? What do I do? I am so sad and confused and I really need help.