How do I invite this girl to a gig i'm performing at?

[john]

Basically i'm performing at this acoustic night at my school - i'll be playing guitar and singing. and I want to invite this girl. the only problem is she doesn't go to my school. she goes to a school just down the road from mine however. We're friends on facebook so we know of each other if u get me. I know that she has friends in my school as I've seen her with them. I just wondering how I can invite her to come, in the most casual, least creepiest way possible. Thanks