“Gosh, I want to go out with you!”
Never stop your flirting, okay? It’s alright to ease back once in a while, but you have got to let her know that you want her (even though you’re not jumping on her right there).
Ask her. I mean it, smile wide and ask her if there’s any chance she might be able to break up with her guy, because you’re madly, psychopathically in love with her! Tell her you want to go out with her!
But don’t wait for answers. Ever. Ever!!
Waiting for an answer would make things uncomfortable. Switch the topic to an entirely new bit within a second each time you pop the magical “I want to go out with you”. That would keep her smiling and blushing, with no hint of being uncomfortable. And you know what, she’s going to think about it after you say bye! Now, we’re talking, aren’t we? You’re going to make her think about you after you’re gone. *wink!*