Hello Hirama666
The "Delete" button only appears when a user does not have any related objects in the database.
For a Teacher user, this would mean you cannot delete a teacher who is teaching a course period, or has created assignments, quizzes, etc.
So the best way to Delete a user is after Rollover, when the account is freshly created for the new year, you can for example remove the teacher from their course periods. The "Delete" button should appear.