My friend is bi! And I am too... PLEASE HELP!

Hey...I'm a 15 year old girl living in California, and I need advice. A few days ago, I found out that one of my good friends -lets call her "Lexy"- is bi. (And I'm not assuming it...I have un-mistakeable proof, and I'm 100% sure that she's bi.) The thing is, I am bi also, but she doesn't know that. I know for a fact that she is only interested in girls for the physical part of a relationship, and so am I. I want her to know that I am interested, and I need advice on how to tell her. Keep in mind... she doesn't know that I am aware that she's bi. She is extremely cautious of who she tells her secrets to, and is constantly fearful of ruining her flawless reputation. (We are both blond, in shape, on the varsity cheerleading team, and are known by literally everybody in school.-four factors that cause you to gain instant popularity.) I've caught her staring at me. When I look, she quickly turns away. The point is, I know that there's no way she will tell me that she's bi. It would be a HUGE risk to her social status and reputation. So... how should I tell "Lexy" that 1.) I'm bi, 2.) I know that she's bi, and 3.) that I'm interested in a sexual relationship with her. THANK YOU.