There's this one girl at my school, once, she looked at me without taking her eyes off me. The first time when she looked at me, I looked at her back and she still looking at me. But then now and then, she looks at me, when I look back, she pretend to do something else... What does that mean?

Her looking away when you notice she's looking at you means she doesn't want to be caught staring at you. Break the ice and talk to her. Ask her a question, like, "What did you think of that homework assignment last night?" [or that test we had yesterday or the teacher's comments on our papers or whatever] If she turns on her heel and walks away without talking, give it up. If she answers your question and keeps talking, encourage her. Girls love to talk and tell you more than you want to know about everything. If she thinks you are a good listener, you might end up with a girlfriend.

Either you have a bugger hanging out of your nose or she would like to talk to you but doesn't know how to start a conversation.