Unfamiliar students remind you of class, but when you see the text message, you have already finished class. How can you come back?

"Thanks to your reminder, I have finished my class on time." Just send a thank-you expression. Although you don't attend class until the other party reminds you, the traditional virtue of being polite to others is to thank the classmate for his caring behavior by leaving a message. Unless you don't want to get this strange attention again, you can end the conversation by directly replying thank you, even if you don't reply.

I suggest you not care too much about other people's opinions. It doesn't matter whether you reply to this reminder or not. Students don't expect your reply when they make this reminder. Just a simple reminder, don't think too much.