How/should I ask this girl out?

Let me just say this because this is what makes me have second thoughts:

Her and this guy we'll just call him "Joe"

Well her and "Joe" have gone to every dance this year as I think friends... And well she is always hanging around "Joe" because I think he likes her but not sure. And with that it takes me to the point that they were/are dating but I just checked on the Facebook and both relationship status say "Single"

But here is my real problem:

I'm not sure if she was flirting with me or not today... I thought I could take a hint if given one but here's what went down. We went on a field trip today and in the back of the bus is where I sat and she sat in front of me and to the right (other back seat) sat "Joe" and about 1/2 through the trip she started to rub her fingers on my knee over and over again and just start laughing... (If you're wondering we're freshman and both 15) but anyway I just don't know because she is really pretty and I'd love to date her but I don't wanna be a douche...

So what should I do?

(I was thinking of talking to her best friend about this matter)