Flirtation tips for a high-schooler?

If I like a girl who I'm friends with on facebook, but I only see during one period during which we can't talk at all, how can I still talk to her more and show that I'm interested? For special reasons which I won't describe here, I'm already pretty sure she likes me if that helps with your answer. Even though I don't want to explicitly ask her on a date yet because I'd rather take things slow, I'd like to know some ways to hint that I'm reciprocating her interest and that I want to talk to her more.