@bobbobbobbob,
It's usually considered a very, very bad idea to date the people you work with. With jobs being scarce, if it doesn't work out, you're left with someone you don't get along with and it's hard to escape. Awkward.
However, you're both 15 so it's not like the restaurant is a major career move for either of you. Even if this is all you do for the rest of your days, the likelihood is that it won't be there.
So, ask for her number. If you're talking whenever she's free, there is at least some interest there. And you should go somewhere outside of work!