It is tough to figure out what other people are thinking. It can be even harder to interpret their actions.
You mentioned that you are bisexual. Do your friends know this? This girl might be nervous or curious. Being a teenager is a tough time. Perhaps this girl thinks you are interesting and wants to get to know you better. She might want to be friends with you, or just get to know you better.
Asking her if she is bisexual might make her feel awkward or defensive. Try talking to your friends about the situation, maybe you have some mutual friends that can give you a better idea. They might have noticed something that you did not.
Just acting like you normally do would probably be the best thing to do. See if she talks to you or approaches you. This way there is no awkwardness and both of you can be as comfortable as possible when you do interact. That is one of the most important things.
Suzanne at AlterHeros