Do you have any classes with her?
That might make it easier to start talking to her.
My current gf had one class with me, and I also hung out with her group of friends after school and thats basically how we got to know each other.
Its kinda aukward in HS to just go up to a girl you've never spoken to...and say "hi".
What i'd do would be walk past her...give eye contact...and be like:
You: "I saw that!
" *smile*
Her: *smiles back* "saw whaaat?"
You: "You were checking me out...again" (shes constantly looking at you)
Something ****y like that will work. She'll just be like "NOOO i wasn't!!" and just be like "SUUUUREE" and keep buggin her about it everytime you see her. I'm tired right now so I guess thats not a good example, but you get the point.
Make it a fun interaction and that image of "fun" will stick with you when she thinks of you. Pretty soon you'll have her msn, flirting with her, and then you guys start hanging out and then you guys are together!
....thats pretty much how I got together with my gf, and she is AMAZING.
PS. A quick tip....befriend her friends. flirt with them (the girl ones of course...unless you swing both waays haha)
Once a girl has her friend's approval for a guy, it makes it that much more likely she will go out with you.
Happened with my gf too. All her friends were like "You guys would make such a good couple, hes so cuute!"
Good luck bro