Does she already know you? If not then, this is your chance to introduce yourself to her. Whenever in class sit beside her if possible. Then just talk to her. Don’t put filter in the way you speak. Just say whatever you want to say. Maintain this friendly vibe for 2 weeks if you meet daily. Then, once you’re already kinda close with her. Ask her out after class. Grab some food or drink. Then, continue to chat with her. Slowly gain information about her. You don’t need to rush to process. Savor it while it last. If she declined to your invitation to go out, know her reason.
After some time, ask her out again. If she continuously reject your invitation to go out, then it means that:
A. She already has a boyfriend.
B. She’s not interested.
If that happens, then move on.