Just pull her aside or something and ask if you can talk to her. Don't act worried, she'll think it's bad news. Say something along the lines of 'I don't know if the reason you said no was because you don't want to endanger our friendship, but if it was maybe we could still give it a try? Because I really like you, and we probably won't see each other much soon anyways. But it's fine if you don't want to.' If you want, you could make it sound a little rushed, so she can see you are nervous (plus she might think it's cute).
Also, if she says no, analyze it. If she sounds unsure and regretful, press gently for more info, there may be other reasons. If she sounds set and determined, just leave it. Say okay, and settle for just friends.
Also, if she says no, analyze it. If she sounds unsure and regretful, press gently for more info, there may be other reasons. If she sounds set and determined, just leave it. Say okay, and settle for just friends.