I see all these people saying go for it go for it .. as if it will be that easy. If you've got the confidence to straight up ask, more power to you and you need not read anymore of this post. I know I wouldn't have the courage.
If you don't hang out out of school, do so. Inviting her over and her accepting is a huge step in the right direction. If you've never had her over before, you might want to also have a couple of her and/or your friends over at the same time and then invite her. After you're comfortable with eachother get close on the couch watching a movie .. don't try some cheezy shit like yawning while putting your arm around her or the popcorn trick.. just sit next to her and if she's comfortable and you're comfortable you'll get closer and closer and eventually (be it that night or a few weeks later) you'll know for sure when the right time to ask is, or better yet you won't have to (the only time this has happened with me was cause I was drunk and we just started randomly making out so no proposition was necessary.)
Good luck and have fun