Hi, I love a girl that's younger than me, how do I talk to her?

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Hi, my name is John. I'm in love with a girl that's 3 years younger than me, I'm 16 and she's 13. I think that she is beautiful, the most amazing girl that I've ever seen, and I also think that she's very cute and nice. We've talking quite a lot via Facebook (almost every day), and she is soooooooooo lovely and friendly and everything. I'm not sure that she likes me too, but it quite seems so. But the thing is, when I'm at school, we never the talk. As I'm the guy, and I'm older, I know that I must go there and talk to her, but I never gain courage to do it. We only say "Hi" to each other, most of the time because she is with her friends and when I see her with them I just get blocked and don't really know what to say, and, as she is pretty shy, she never talks to, but she is always saying to me via Facebook that I must talk to her, and that it seems that I don't care about her at school and all of that. Can someone help? Thank you, I really love her.