Social networking sites are ace to find out whether or not someone is in a relationship. Search for her on MySpace or Facebook or whatever you're into (or whatever network she's on). Easiest and most painless way to find out if she's taken.
Anyway, if you're just looking to dip your , just make sure you bear in mind that she might not be up for a relationship like that - but by the sounds of it, everyone at your school is sexing each other, sounds like she'd be up for it too.

As for advice:

I'd wait for the new school year to begin.
Then I'd strike up a convo about what she's studying, football, or whatever.
Ask for her number, if she asks why say it's in case you get stuck with homework (or something along the lines).
You now have her number - use your imagination for how to use it.
Meet up at a park, or before/after a football match, etc.
???
Profit (your definition of profit, mine would be a loving, lasting relationship with sex only a small part of it... yours is clearly different. )