Option 1: Delete
If you're comfortable they've left and you don't need their data then you will want to delete their accounts. To do so please:
Go to Account > Classes
You can delete whole classes by pressing the Delete button on the class row.
You can delete individual pupils by ticking their checkboxes and press the Delete tab at the top.
Option 2: Keep them and move them to a "leavers" class
You could keep their account so that you still have access to their data and if the pupil's new school uses TTRS their account can be teleported over.
To move pupils to a leavers class please:
Go to Account > Classes and tap any class containing pupils who are leaving-but-you-want-to-keep.
Select the checkbox next to the names of these pupils.
At the top of the page, press Move > Move to Math(s) Class and follow the steps.
Any problems? Please email firstname.lastname@example.org and we will be happy to help!