Ask her out "as friends"
Take someplace non-threatening, like a coffee shop and just talk and flirt with her.
DON'T use corny, bullshit lines like "I wish you didn't have a boyfriend..." "If only you were single..." "God you're so beautiful..." or "What about you and me?" In fact, as a rule of thumb, don't utter any phrase that isn't followed by another one, unless it's a joke.
If you wait for her reaction, you just used a line.
Tell her honestly and simply that you're very attracted to her, you like spending time with her and you feel like she flirts with you too. You aren't trying to break up her and her bf, but you don't want to miss the opportunity to date her.
You won't look like a couple-wrecking asshole, or a melodramatic douchebag. If she's interested, she'll take you up, if not, move on. Don't think, however, that her having a bf locks you out. After all, she's in
college and he goes to another school, so they've probably been dating since high school. I wouldn't be surprised if she was ready to move on.