I’m assuming you’re not the teacher.
You want to catch her when she seems bored or otherwise not busy and talk to her. Doesn’t really matter about what. Just find a reason to talk to her and go for it. This is called “breaking the ice.” The next time you talk to her it’ll be easier, and you’ll have more presence of mind to try to get to know her.
Do not ask a girl out in the same conversation you’ve broken the ice in.