Need help with a girl, this is my last resort?

Ok so this is a very weird situation. There's a girl I like but I'm too awkward to spark conversations. It's weird because I'm very out going in the class we have together, I'm the class clown and I've actually talked to her a few times before. She laughs at all the idiotic jokes and blatantly stupid things I say but I just get to nervous to approach her. My best friend hates her because she doesn't let him cheat off her tests. This may seem minor but the last time I almost had a girlfriend he helped set me up. I'm a senior, she's a junior but I've never had a real relationship in high school. I'm apparently more popular (but I feel like a loser half the time) and I feel that's made her uncomfortable the few times we've talked. I've seen her look at me in class before and but I'm not good at picking up signals girls give off. I've had girls "like" me in the past year or two but I either didn't like them or I wasn't comfortable talking to them. I think this girl is different because I've talked to her before but I just never seem able to muster up the courage to just go talk to her. Does anyone have any advice they could give or have any clue how I can solve this ongoing issue I've had?