Most Helpful Girl
She's looking at you because she's interested (or she thinks you look familiar/weird/etc). She looks away when you notice her because she's a GIRL LOL, most of us get anxious when a guy stares right back. Maintaining eye contact with a guy we don't really know but think is cute is hard. I've TRIED to maintain eye contact with guys I think are cute before but the sense of pressure really builds until I just have to look away.
You should go say "Hi" to her.
One time, I stared at a guy until he said hello to me but that time, I was staring because I was trying to place where I know him from.