My classmates joked that I was someone's girlfriend, but we weren't. And when he said this, the teacher just came over. I don't know if he did. ...

I suggest you make it clear to your classmates that you are not boyfriend and girlfriend. I hope she won't misunderstand you next time, and joking will be a joke. Then, in your usual study and life, you should try to keep your distance from "someone" and don't let everyone really think that this is a practical matter, which will be difficult and embarrassing ~ ~ In fact, the teacher doesn't care much about these topics, as long as your grades remain stable, it will be OK!

thank you for asking. I wish you a happy life and progress in your study!