You're just infatuated by the first girl that comes your way. That's okay, I was there too.
The truth is, she liked you and you didn't do anything about it. This doesn't mean you can't save this type of relationship. You got to reconnect, say "Hey, I haven't seen you in class for a while, is everything okay?"
Whatever bull crap she says is irrelevant, then say "That's too bad, I miss your company, when you free for lunch/dinner?"
...more bullcrap talk, at the end of the conversation say "Good, then it's a date then. Can't wait to see your pretty face

"
Kino escalate during the date. Make sure to kiss her or at least attempt to. If she lets you, then make a stronger move (ie. try to finger her or sex).