SHE IS TRYING TO STEAL MY BOYFRIEND!!!!!?

ok what should i do there is a girl in my cl who is trying to my bf and its driving me crazy she keeps flirting with him and giggling and twirling her hair I HATE IT since they sit next to each other they can talk alot i sit right behind them so i have to watch, listen, and stare at it the entire period HELP ME what should i do to stop all this?!?!?!?!