Try to strike a conversation with her. If you see her sitting alone or getting bored, go over to her and chat. She is likely to appreciate it and find you both friendly and nice, two of the most important things to make someone feel comfortable around you. You can also join the group she is talking to.
Flirt if you must, but do not come off as desperate. Keep your flirting as casual as possible. If you find her looking at you, smile. Look for opportunities to make eye-contact with her. Be careful not to stare at her for too long, as she might start finding you creepy.
Look for opportunities to do her favour. This can be anything, from offering to carry her books for her, helping her pick them up in case she drops them, rushing over to the canteen to get something for her to drink or eat, lending her your pencil in case she does not have one, offering her your notes, etc. While doing her favours, make sure you do not end up becoming her slave.
There is nothing a girl likes more than compliments. If you really want to know her and make her feel comfortable in your company, send compliments her way. Say something about the clothes she is wearing, or the way she is wearing her hair. Compliment her looks, especially her eyes.
Make her feel special by walking her to her locker, lending her your umbrella when it is raining outside, offering her your jacket on a cold day, or consoling her while you see her upset and close to tears.
Always remember to be yourself around her instead of pretending to be someone you are not. She, or any other girl for that matter, will never appreciate a person who is trying to be someone else.
Try to get into her social circle by chatting up with her friends. They may serve as an important source of information, while a few of them may actually help you to get closer to her.