If your school was closed for a half day, you can record this by setting a "school closure" register code to apply to all students for just the one AM/PM session. To do this, head over to Register > Manage Register:
Once the interface has loaded, locate the "Date From" and "Date To" boxes. Set both of them to the date of the half day. Finally, select the AM or PM option to reflect which half of the day the school was closed. Now click View Register.
A full rollcall of all students should load, with just the one attendance session showing next to each name. Tick the box below the heading titled "Select" in order to check/uncheck all of the boxes in the righthand column. This will ensure any changes you make will apply to every student's entry on the list:
Scroll to the bottom of this table and from the "Choose Code" dropdown menu at the end, select the required "# - School Closed to All Pupils" code, and then tick the "Overwrite" box. Finally, click the "Fill Gaps" button to set every pupil's attendance code for that half-day as "School Closed to All Pupils".
Your list of students should now have the "#" attendance code recorded for the just the one session. Note that a new button will appear at the top of the list which allows you to undo your last operation. Click the Remove Last Insert button if you wish to revert the list back to its previous state: