As this thread suggests, if you're interested in a girl: go for it. There isn't a magic 8-Ball that will truthfully tell you if a girl has an interest in you, so the best way to find out is to ask her to a dinner & a movie, or something like that.
If she's already a good friend, it's best to make it something out of the ordinary, so it doesn't seem like a "friends hanging out and having a good time" thing and forces her to realize that you want it to be a date. I never took those chances in highschool and waited quite a while to have a girlfriend - and it's not worth it. Take chances, ask out the girls. What's the worst they can say, no?