Should I give up on this shy girl (a junior)? I (a senior) am really considering moving on.?

There is shy girl in my science cl that I like. We talk occionaly. I asked her out to a football game twice, but said that she was busy over the weekend and doesn't like football. Then I asked her to go see Harry Potter with me and my friends, but she said she was going to see it on opening day. I also told her how I feel over the phone AND then more recently in person at her lunch table where her friends were. Both times she told me "not really" when I asked her if she liked me after I told her how I felt about her and told me that "she didn't like me in that way". The thing that gets me, though, is that she always glances at me for about 3 seconds every couple of minutes in science cl. She even does it when she sees me walking in the hallway, at the NHS induction we had, she was just staring at me once in the cafeteria when i was walking from the lunch line, at APES club meetings, and when I'm visiting my friend's first period precal cl in the morning. I even changed where I sat in cl to see if it was possibly a habit she has, but she still looks straight at me when she thinks I'm not looking. She does laugh sometimes in cl when I try to make jokes. She also got kind of mad after after I flirted with a couple of girls in class one day. She evens sits in the same positions as me sometimes when she sees me sitting that way in class. I am so confused about this girl. (When asked by phone 2 months ago why she looks at me, she told me she just "likes to look around the room".) In reality, she definately does looks my way predominantly though. Note: She looked at me alot in class even before the first time I asked her out. Please help! Thank You. Today in class even, she stared at me alot (when I was looking down or away, I have good periphreal vision) and would look away really fast when I would look at her when she would be staring/looking at me.