You’ve had a crush on her since 6th grade and you don’t have evidence that she has a crush on you, too? Either she doesn’t feel the same as you do or else she does and has elected not to tell you. Let sleeping dogs lie and continue to be a good friend and a young man she has reason to admire. There is no good in telling her just yet.
When you think you are old enough to ask classmates to a dance, ask her. Do NOT tell her you have had a crush on her since 6th grade, just ask her out when you think you’re old enough for that kind of thing. Then see what happens. If she says, “no, thank you,” and all you’ve done is to ask her to a dance rather than suddenly confess what could sound to her like a five or six year obsession. By holding back, you will have at least preserved the friendship.
If you two decide to get serious and you confess that you’ve had a crush on her since 6th grade, then she’ll find the revelation sweet, charming and flattering, rather than scary. Never confess what sounds like an obsession until you’ve established yourself as a normal person in a relationship. When the crush has turned into something real, then you can let the cat out of the bag.
If she confides in you that she thinks no one will ever want her, it is OK to say, “c’mon…I met you in 6th grade, and I thought you were the prettiest one. Of all the girls I know, you’re one of the ones I know will find somebody. There is no question in my mind.” Use the past tense, though, until you are on good ground for a real relationship, or until she says, “Well, would* you *date me? Because I don’t want to find ‘somebody’ unless I can find somebody like you.’” Then you can say, “I promised myself I wouldn’t start dating until I’m 18, but if you’re still available when I am, then yes, I would date you.”