Why can't I change the attendance of an old class any more?

Because an instructor can take or correct attendance only on the day of the session and for the following 7 days. After that, only the school owner can, with no date limit.

1. Contact the school owner and give them the class and the date.
2. The owner opens Attendance (or Schedule), taps the session, then Take attendance, corrects it and taps Finish.

If the session was waiting in the device's send queue, it becomes "Not sent": tap Remove once the owner has been told. Message shown: "This session is more than 7 days old: only the school owner can still change its attendance."