How do you get a girl to admit that she's bi?

My friend and i were at a sleepover when we were dared to kiss, cuz everyone says im bi(i kinda am...) and she kissed me. Well, then everyone was daring us to: kiss, lick, make out, have etc etc etc. Well, after a while, we just were having for fun. All the other girls were too. The next time we were at a sleepover together, she was like: "Im not bi!" We all were like, you had WITH A GIRL! She claims it was all a show. But then she was having with me when we were studying. But still: "Im not , or bi! Geez" I know she is cause she comes over just to kiss me the whole time, but she refuses to tell anyone. Not even ME! How can I make her admit it??