How long should i wait after meeting a girl to ask her to hang out?

Alright, so I'm a senior in high school, and my entire high school time has been focused on athletics. I've decided it's about time that I get myself a girlfriend, and there's this girl that I've recently become friends with that i think I'm starting to like. Since freshman year we've been "acquaintances", in that we had a few cles together, and would occasionally chit-chat, but not much. This year however, we have two periods together back to back, and we've started to do quite a bit of talking. I've noticed that when we work in groups or anything like that, she tends to stand fairly close to me, and occasionally make some sort of slight physical contact. I don't know if I'm reading too much into it, but I'd like to ask her to hang out sometime, but I don't know how long I should wait, or what the level of friendship needs to be before i, say, ask her to a movie or something like that. I don't want to wait for months though, because it's senior year and I don't want to wast time, and I'm kinda paranoid that someone else may beat me to it... thanks for any help anyone can give.