Girl Problem?

I am 24. I like a girl at my cl. But she always sits in the middle of her two friends. I feel she likes me but she is not trying to understand that i m unable to talk to her cause i cannot sit beside her. Once i tried to sit beside this group and had a nice conversation with her friend but not her. She also sends signals that she wants to talk to me and she once asked me to take care of her seat when she was away and once when i was sittin in d row behind her she asked me what was written on the board instead of askin her friends sittin beside her, she turned around and asked me. I do not even know her name but i know her friends name and we even greet each other. What do i do to get close to her and make her my girlfriend??