Honesty.
Whatever else you do, you MUST be able to live with it later - what if this girl turns out to be "the one"?
If you lie to her to get an introduction, you might well be totally fucked if you ever tried to get serious.
Personally, I'd go with what you said in post one - something like
"I've been watching you come out of this class, and you (are really hot/interesting/have spectacular breasts/should suck my cock/whatever)* so can we go for a (coffee/donut/lunch/anal sex session while your mates piss on me)* or I'd never forgive myself"
* delete as appropriate.
