When someone has left the school it is the schools responsibility to ensure the user account is disabled to prevent the account form being used.  It is simply a matter of removing a tick box.

You will need System Manager access.

Note: The pictures below are from a test database that does not contain any real data.

Open System Manager

Ensure the filter for Active says Yes

Click on Search to view the list of users

Double click the user you wish to make inactive.  In Login Details (section 2) remove the tick for Active.

Whilst in the record, it is good practice to set an end date to the groups.

Select the group, click Remove and answer Yes to the question.

Once done, click Save at the top.

If you refresh the list (you may have to click on the Browse button if the list is hidden) the user will no longer be visible.

Why make inactive?  Why should I not delete the account?

By making the account inactive, you are keeping a record of the account for audit purposes.  It will maintain the user account (linked to the staff record) and the date range the user had specific group access.