When I was in high school, my "girlfriend" was totally okay with me pursuing other women. She'd even ask me if I had any luck with a specific gal, and gave me the classic "better luck next time" when the inevitable failure response came up. She just wanted me to be happy.
Ours wasn't a real relationship and neither is yours. Your current girl shouldn't be a factor in your decision to pursue your old crush. There's no point in having a relationship for the sake of having one. Passion is an essential ingredient and you lack it.
As for the matter of pursing your old crush, it's basically an issue of pride. Emotionally immature people (which is what most teenagers are) can be painfully wishy-washy with their decisions. I expect that if you try anything she'll just change her mind again. I spent my entire high school life going after one chick because she kept giving and withdrawing her affection at strategic moments to keep me perpetually hooked. This wasn't some calculated thing, it was just the way her mind worked.
I got more from her than I would have if I did nothing, but it wasn't worth it in the end. You need self-respect. You need to say no to people who try to play games with you rather than treat you like a fellow human being. I know society tells you that games and mind-reading are just how dating works, but it isn't. There are people out there who will express their feelings plainly, or give you a fair shot if you make the first move. It may not be exciting as this constant mysterious back and forth of "will she or won't she", but it's a lot less complicated and a lot less anxiety inducing, and in my opinion, creates a stronger foundation for a relationship where both partners are supposed to be equals.
I don't know how your classmates would react to that approach, but if you're at the point where you're asking for dating advice on the internet, then high school probably isn't the time your love life is going to bloom. The guys who get laid all the time are able to do it because it comes naturally to them. They don't think, or scheme, or plan, they just do.
Wait and see what happens, but don't be upset if that happens to be nothing. There is more to life than love and sex.