Why is a girl with a boyfriend act like she's interested in me?

Im a senior boy in high school, she's a junior that transfered to my high school last August. She got a boyfriend who's a senior and I know him because he's one of my friends. But the problem is that when I see her in class (I sit right next to her in class and I only see her during that hour), she always acts like she's interested in me. Moves her body towards me acting like she is just working but in reality she is looking at me (I can spot her sometimes but usually she's quick about moving her eyes down). Asks me stupid questions that she already knows the answer to. Keeps fiddling with her hair when I'm next to her. Copy some moves that I do (like crossing my arms, setting my head on my hand (when its upright not me leaning down on it)). She always laughs with me at my stupid jokes that no one thinks is funny, acts completely shy around me (given too she is highly shy around everyone but when she is next to me its like she's shyer than before). It puzzles me because I did ask her to prom but she said she had a boyfriend... Okay but I did say if he didn't say anything like "You wanna go to prom,," would I be her first choice and she immensly said yes.
Point to this is what is going on, does she really like me and not much of the other guy or what? I would love to take her to prom but she already has a boyfriend so she has to decide basically. I need help understanding whats going on.
Why is a girl with a boyfriend act like she's interested in me?
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