If a student is showing as not "registered this session", on their student profile, but you know the student is actually present today, this is probably down to an attendance code register error. You can correct this by heading over to Register > Manage Register:
Select the student you require from the Single Student dropdown menu, set today's date as the date in the Date From and Date To boxes, and then click View Register:
From the Choose Code dropdown menu, select the appropriate code (e.g.: "Present / and \") and, if you need to overwrite the existing entries, make sure the "Overwrite" box is also ticked. When you're ready, click Fill Spaces to change the attendance code data for the selected sessions.
The student should now be showing as "YES" under the "Currently in School" heading: