To bulk fill an attendance code for a group of students go to Register > Manage Register. Select the Form from the dropdown box, and select the date you wish the code to apply to. To the left of the screen click on AM, then View Register.
A table will appear with all students in that Form and the AM register area present for codes to be input.
Ensuring each student has a tick in their Select column, go to the base of the table and here you can bulk fill with a specific code from the dropdown box.