How to control myself

Theres a girl in my class who sits behind me with her friend. She is actually my very close friend who I know since childhood. Whenever I drop something on the ground and try to pick it up my eyes always roam under her skirt giving me a b****.We even once shared an umbrella but decided to nit to use it and have fun and since she was in her school clothes her bra was visible. I always had to j*** off to control my feeling. Should I just ask her away and risk away our goid old friendship fir some fun or just keep quite. If you can help pls comment