If you go to Register > Manage Register, then enter the day the school is closed in the Date From and Date To fields:
When you click on View Register it will bring up all the students in the school. At the bottom of this table, you will find a field where you can enter a specific code you wish to use for all students that day (e.g. #), select whether you wish to overwrite any existing codes input for that day, then click Fill Spaces.
You will then find that all students have been applied this code in the registers.
