Give Her Direct Eye Contact
If you have glanced over and noticed her staring at you a couple times, stare back with a smile.
If she smiles back it is a good sign. That does not necessarily mean she wants to date you, but it does mean she is not repulsed by you. If she frowns and quickly looks away, you just might have something on your face.
Strike Up a Conversation
Come up with an excuse to talk to her, such as discussing school work.
If she does not seem to be in a hurry, take it as a good sign. She may seem to be listening intently while smiling up at you, tilting her head to the side and twirling her hair or may be looking away nervously and turning red. Both are strong signs that she likes you. End the conversation with something like, "It was good talking to you," and if she says the same, that is another good indication that she likes you.
Make Physical Contact
The next time you are around her, playfully nudge her shoulder if the two for you are joking around.
She may playfully nudge you back, which is another indication that she likes you. Or sit down next to her in the cafeteria, brushing her shoulders with your own. If she does not move away and she seems happy that you have joined her, that is another big sign that she likes you. Take it a step further and grab a french fry or something from her tray. A girl who likes you will not get mad.
Ask Her Out
Take the plunge and ask her out.
There is no better way to find out if she really likes you. Wait until you two are alone, perhaps between classes or after school so you do not have to try to get through a bunch of her friends to talk to her. Keep it casual asking her to do something you both are interested in. For example, ask her if she wants to check out a scary new film if you know she likes horror movies, or if she wants to check out a band playing at a local festival with you. If she has been giving you all the signs, she will say yes and you have got your girl.