How to ask out a friend?

[Brendan]

I have been debating wether or not to take this girl to a concert, I'm a senior in high school and we have been friends since middle school but not really close friends, anyways I decided to grow a set and ask her but the thing is I'm Used to asking girls in my league but she's out of my league how should I go about asking her without making it awkward

It's a yellow card concert and I think she likes them and I'm going to pay for her ticket I also don't see her in school to often maybe like once a week and I don't have her number anymore is Facebook an option