How can I start talk to a girl that I like if I'm shy?

Hi people on loveforum - Can you give me advice please? Thank you! =)

There's a girl that I like and I think she likes me as well. We're both 15. She talks to me or about me in a really accusation manner sometimes, like she was lightheartedly making fun of me playing cricket in PE, talking about my cricketing skills and stuff, but she wasn't really serious and was just kind of teasing I think. (She also said "you know I'm not serious right" afterwards too). She also used to always make me take the "form folder", which is the folder containing who was present and who isn't and it is generally a considered a burden to walk to student services. She also used to laugh or giggle with her friends when I said "Hi" back. So she definitely likes me right?

The thing is I don't know how to approach her. It was weird with her signals, because for a week she seemed to be sending positive signals, then she seemed to be ignoring me, now good signals again (I think). Might because I don't know what to reply to her and she gave up? I just want to say "Hi" to her, but its so hard because I only have like one class with her and she's always with friends. I don't think I can join in their conversation or spontaneously approach and greet her while she's talking with her friends right? Or am I too shy? A few times I get to see her walking alone down a hallway or something but my reactions are slowed even more by being shy and I can never get myself to just say hello. Like today I was walking close to her for about half a minute but I just feared my voice wouldn't be clear when I broke the silence so I was too nervous to speak. So what do I do?

I'm not like a complete loner in our school, but I don't have as many friends as her and I feel greatly dwarfed by her and her friends' social skills and friendship groups. Honestly I feel kind of amazed that she would like me sometimes because she seems to be so much above me. But maybe that's just me. It feels really embarrassing sometimes when she communicates with me in any way and I don't how to respond, e.g. she makes me take the form folder > I really want to reply with something that means "I like you as well" but what could I say? It's not a direct statement or anything.

So I need advice on my situation please. Thank you!

By the way I'm currently waiting to talk to her when she goes on FB chat.:D