How do I ask her out considering the cirstances?

Ok so the deal is I'm in grade 11 shes in 12, graduating this year in 2 months, we were in the same social and bio cl together for a semester and we became friends, weird friends though if you know what I mean, like we don't really talk outside school. After awhile I kind of asked her out to a dance and to a movie and she said she would love to but was busy and blah blah and I didn't ask again, then the semester ended and I barely ever saw her again. I had really really liked her and I was always getting down about it around my best friend so he talked to her and let a little bit on about me and suddenly we started talking again, still not alot but comfortable conversations (the only thing is I always have to start them and if I don't keep it going it ends) so I told her we should chill on the weekend and she said she would clear her schedule, I told her I had some free movie tickets and then we could for lunch or dinner or whatever and to keep it from being too awkward I asked if she wanted to bring any friends and she said just you. It wouldn't be a problem aside from the her being older part (her being able to drive and drink and all that while im still stuck back at 17) and the fact that shes gorgeous and I'm... well just another guy, a little built nothing spectacular so its just adding to my stress, I don't want to jeopardize our friendship before it even has a chance to rekindle itself but the last time I waited to ask her out she got stolen right out from under my nose. So case in point shes older, and beautiful, im younger and nervous as hell, and our relationship is just restarting itself, so what now?