You should play it cool. Don't start acting like a geek. She told you she has a boyfriend flat out. So you flat out don't ask her for a number right away but you play it cool for a bit and she will defintley know that the game is on. This is what seduction is all about. This is how you should say it after you finish classes:
" hey I am going to get something to eat at a (Bar - Insert name here) down the street from here, you are welcome to join me".
If she says yes then its game on. You order some nachos some drinks, have a few jokes and laughs, kino her, play a game of pool or darts, and if everything goes well you either go for it and kiss her or just ask her for her number.