My bestfriends girl likes me:S?

So me and my cl were at a concert band preformance and after we got back i walked by her and said bye and she said wait a second can i talk to you for a second and i was like yeah sure what about and she was like in private so we go behind a stair case(and keep in mind the school is almost empty after coming home at about 10:30) and after theres no one there she puts her books shes holdin on my chest and begins to kiss me. i didnt know what to do so i stopped and was like woah what are you doing because her boyfriend whome shes had for 2 years april 19 is one of my best friends and she said shes been thinking and that she really likes me and have for some time now so we began to makeout again. i told her to think about it over the weekend and that brings me asking this question. what should i do? i realy like her to shes soooo cool and jsut like me in everyway and ive noticed her not paying that much attention to her boyfriend(my bestfriend) the past couple weeks. any sugestions on what she or I could do?