As a Teacher, you can have Moodle automatically mark students as absent after a session is over. As an Administrator, you can set this as the default behaviour in the Attendance plugin Settings, so perhaps your administrator hasn't done this for you.
As a Teacher, begin by checking the "Status set" and identifying the default status (see screenshot 1 below: status "Absent" is probably what you want. Then, when adding a new session, expand the Student recording tab and select "Set unmarked at end of session" in the "Automatic marking" field (see screenshot 2 below). This tells Moodle to set the status for any students who haven't registered their status to whatever the default status is. Moodle runs a periodic batch job to set the attendance status.
If your session is 60 minutes long, Moodle will wait until after that before setting the status. I think you can also specify that students must record their status in say 30 minutes, even if the session is 60 minutes, so I presume Moodle would set the status after 30 minutes in that case.
