Girlfriends best frind is trying to break me up?

My girlfriends best friend hates me. When i am around my girlfriend shes nice to me but then when i sit next to her in art she is al total jerk to me. My girlfriend is always sticking with her and im sure shed get really pissed if i talked to her about it. Her friend uses me to get to get by things and is trying to creak me and my girlfriend up. I don't know what to do