Search Results: asking a girl to prom

Okay, so first off, this is going to sound extremely superficial (I cringe while typing this), but here's my situation.
Some background info:
I'm a junior in highschool now. I have already gone to prom twice (I was asked to prom by seniors during my Freshman and Sophomore years), and now, I'm finally allowed to go on my own accord (our school has junior-senior prom).
So there was this girl in my grade that had just transferred in this year (for anonymity's sake, I shall bestow the alias of "Anna"). I was interested in her, and I asked her out within the second week of knowing her. She said no, but not because she didn't like me, rather that she was still adjusting to the school. Over the five or six months I've known her, she's shown some moderate amount of interest (or at least that's how I've percieved it) in me. So, I decided I'd ask her to prom.
I consulted with her best friend (whom we shall bestow the alias "Rebecca"), who happens to be a Freshman (who honestly looks and acts like an upperclassman), and she said for me to go for it. So I did. And she said no.
So, here I am, prom's a month away, and I have no date. There is literally no-one else in my grade that I would even consider taking to prom (I go to a private school where 90% of the girls there are preppy sluts).
Now here's my dilemma. I've been moderately interested in Rebecca for a while, but I've held back on that, because of the age difference (3 years isn't that much when you think about it). So I'm thinking of asking her, but I'm not sure how appropriate that would be, considering that I had just asked her best friend.
Should I do it, or not?