How to tell a girl, you don't hate her, but you don't want to date her.?

Someone confessed their love to me. Said they liked me for several years and cited specific examples where I stood out in those several years. I know the person, and sit by them in a class. She's not a whore, slut, or even stupid. She's shy and quiet. I don't know what the hell to do. She doesn't talk to people outside her group, and even after sitting by me (her precious crush) she still hadn't opened up. My life is too controlled by my parents to have a successful dating life (we're both juniors in high school) and personally, I would value a girl friend over a 'girlfriend'. I don't hate this girl or creeped out, I'm just stumped on what to say. I didn't reply to her confession, as it was on paper, so what should I say? I don't want her to cry or anything. I don't want it to be awkward or make her hate me. How do I go about telling her I want to be friends?