To remove a User from your school, go to the Settings > Users page.
Double click on the User that you would like to remove from the school to get the following dialog.
Press the remove button located towards the bottom and select delete on the confirmation dialog.
Note that removing a User does not delete their account from our system in case they have access to other schools. It simply removes all access to your school.
The operation is instant, they are currently not notified.