Answer:
Student B: Hey, have you seen Student A recently?
Student A's Friend: Yeah, he returned to school last week. He had been absent for more than three weeks.
Student B: Really? I didn't even notice he was gone. Why was he absent for so long?
Student A's Friend: The homeroom teacher had discovered his absence, and by the time he came back, the teacher had already reported it.
Student B: Oh, so the teacher found out about it before Student A returned?
Student A's Friend: Exactly. The homeroom teacher had reported the absence, and only then did Student A come back to school.
Student B: That's quite a long absence. I wonder what happened.
Student A's Friend: Yeah, I'm not sure about the details, but maybe you can ask him directly to get the full story.