For teachers, family members, and students, just contact the admin to get your account deactivated.
For admin, go to the admin dashboard and click on users. This will show all the users in your school. Search for the person that you have to deactivate, hover over the name, and click on the "x" mark at the side.
You'll be asked to confirm if you really want to remove this user from your school. Click on "Yes, remove the user" and you're done.