Go to Register > Manage Register. In here choose your date range, year group or single student, select AM or PM (or default to Both) then View Register.
From here leave all students ticked or untick student as needed using the tick column on the right hand side.
At the bottom of the table you will see a "Choose Code" dropdown, an "Overwrite" tick box and a "Fill Spaces" button.
Choose code - This will allow you to choose the code you wish to bulk fill.
Overwrite - this is to be used if you wish to overwrite existing codes, with the new code you are about to bulk fill.
Fill Spaces - This button is to fill the red boxes (or existing codes) with the code you selected from the dropdown.
This is now updated.
Please make sure you only have the date range that you wish to bulk fill open on your page, as this action will update all ticked students and all dates shown.