When a pupil leaves the school, you should archive them so they don't use up your licenses. Their account will still be active, but will no longer have full access. Parents can then choose to add a subscription to their account by following the steps in this article.
In order to archive a student:
Login to the Teacher Dashboard
Select Manage > Students
Select their class from the drop down at the top of the page
Select the student you wish to archive by click on to tick box to left of their name
Then select Archive in the Action box and select Apply