Having been a psychology major in college and a lifelong social scientist, I might have some help for you, youngster. As everyone knows, chicks dig confidence. Not all confidence, mind you. Misplaced confidence (such as being vain or proud to be a giant tool) will not work in your favor. Take some time to figure out who you are, what you want to do and what your strengths are. But DO NOT brag about it! That's the first step to Tool Academy. Try being humble and funny... you'd be surprised how easily a chick will relax around a funny guy, and don't be afraid to make even a stupid joke. If the joke is bad and she still laughs, that's a sign that she's willing to forgive the bad joke and admire that you tried. Besides, the upshot here is that you made her smile, and women don't often forget about guys that can do that to them.
You should play the shy card for a little bit, hang around but don't make yourself a bother around her or her friends. Pay attention and learn what she likes, what she talks about, who she talks about, what music she listens to, what television shows she watches, etc. Do your homework and at least pretend to take an interest in what she likes (if she has different likes than you do). Common interests make for easy conversation, but unfortunately those conversations often are short lived. If you both agree on something, then what the hell are you going to talk about? If you find she has an interest in something you cannot tolerate (such as watching Jersey Shore), then that would easily be a topic to avoid if it is going to be a point of contention. Try to find something that, while you may not find it interesting, you can at least discuss without it creating an awkward conversation. Don't be afraid to say something nice or sweet, even if it sounds like a really corny line. Even if the line doesn't land in the "sweet" zone, you could still get a laugh out of her, which is still a positive response.
As you already mentioned, she is currently dating someone else. This isn't exactly a negative thing as you can make this work to your favor. This takes the pressure off of trying to impress her right now into trying to inspire romantic interest in you. Listen to her talk about her boyfriend (often while resisting the urge to compare yourself to him) and learn what she doesn't like about him. If you share a similar behavior she doesn't like in him, be sure to NOT do that around her. She certainly isn't going to consider "trading up" if she finds herself in a similar situation in the long (or short) run. You'd really be surprised what you can learn just by simply listening without saying anything. Smile and nod a lot, just give her plenty of room to talk. Be sure you pay attention! It is pretty simple to figure someone out because often they will casually tell you something that could really be a positive learning experience for impressing them later.
I agree with a lot of the other advice about talking to other girls, but more importantly you should develop more of a comfort level in just talking to anyone. People are people, regardless of their sex. Getting an email or messenger contact is important because people are a HELL of a lot more open to talk about themselves when they don't fear judgement (such as a face-to-face conversation when they can see your physical response). You could potentially find yourself unlocking a wealth of information about her that way if you can email or IM each other as you'll get a less censored version of a conversation. I wouldn't worry too much about her friends... people form opinions in high school simply based on the way you look, especially if you don't conform to the latest trends of the day. If you can show this girl what a good guy you are and impress her with your ability to listen and converse, she'll do the work of convincing her friends what a nice guy you are. This serves a dual purpose of spreading a good opinion of you, and if she doesn't find herself attracted to you, you've already got your foot in the door for one of her friends. Granted, it may not be the prize you are eyeing, but at least you don't go home empty-handed. Also, if she just doesn't feel the time is right, then if you date one of her friends and impress this other chick, then you still improve your overall standing within her circle about what a great guy you are. When things might finally evolve to where the girl is single again, then you simply need to find a way to break up with the friend of hers (unless you find yourself in a better situation anyway, then no point in ruining a good thing) in a peaceful manner. You sure don't want to come off like a jerk just to dump the broad to go out with her friend, that will seriously negatively impact your chances.
So take your time... even if she breaks it off with this guy, it only provides you with an opportunity to move closer to her by being there to support her, but do not start to sell yourself as a better boyfriend than the last guy. Instead, enjoy this chance you have to learn about her and study her likes and dislikes. Remember to look past the physical person and figure out if you like this girl for who she is rather than what she looks like. Nobody likes a shallow person. I am serious, take notes if you have to in order to learn about this girl (just don't be a dumbass and write things down in front of her!) so that you don't forget things. Once you have secured a place in her circle of friends, you might get a FB ad without petitioning for one... and that would certainly be a good sign. And if you make it to the inner circle, then you can still take her out (as friends) to the movies or whatnot with maybe one or two of her friends, just make sure the female to male ratio works in your favor. Holy crap, never offer to go out with her if she is going to always drag her boyfriend around. It is understandable to do it a few times, just to size up the competition. Anything worth having is worth the time you need to invest in order to achieve, so you don't want something that is an easy conquest. You'll be a lot less happy if she is easy to get as it won't leave many surprises over time. If she isn't the girl you want, then take away the lessons you learned to improve your distinction for the next girl you are after. Don't be afraid to look like an idiot, chicks actually seem to think this is a good thing. Shows you are leaving yourself exposed and aren't trying to pretend you are someone you aren't. Being genuine and honest about being a weirdo or an occasional dumbass puts people at ease. You don't want to come across as someone who is better than anyone else as that is the reverse of being humble. Don't forget to compliment her at least twice a week... you really don't want to overdo compliments or otherwise that is all she will see you as is just someone to make her feel good. You want to impress her, but keep her curious enough to maintain interest in you as you are in her.
Sorry this was so long, but I really hope you take away some good advice here. Stay positive and try to keep away from exposing bad habits too early. Have a little fun with her to keep her grounded. If you appear to be putting her on a pedestal, then she'll just want that attention all the time. Encourage her to be more herself around you, it should increase your learning potential exponentially. Good luck!