Just open up and talk. You have a common theme and that is class.... A majority of people's problems out in the world is they feel like they are intruding on someone else by talking to them. Out at a bar, you find something to talk to a girl about. Then you develop conversation over topics and common themes.... In your case, you already have that common theme aspect.
Who cares what you say. Just say it with confidence and just talk. If you truly are at a loss, ask how her day is. I wouldn't go with that, I'd try to be more creative, but who cares... Compliment her on her shoes or shirt or whatever (not her looks though).
Poke fun at her too. Be friendly, but flirtatious.
Years back when I was in college... I was sitting in my class and some random girl who I never spoke to passed me a note making fun of what the professor was wearing. It struck up an entire conversation and we ended up hitting it off. Anytime I saw, I'd give her my cute "i'm sexy but mysterious" smile and she would smile back and talk with me.
You already have that foot in the door, so go ahead and take advantage.... Best thing to do, ask for her opinion on something because we all know girls love giving opinions after opinions on anything and everything.