The mayor of Copenhagen has initiated a campaign that would lead to placing those who head schools to be held responsible for the success or failure of schools. As one who has taught teachers for over fifty years let me support this program. It is the job of a principal to supervise members of his staff. If they fail to do their jobs, it is the responsibility of a principal to provide support, further training and opportunities for continuing education. If a teacher is incompetent, and many are, it is the job of the principal to either assist them in becoming effective teachers or to dismiss them and get a different teacher. The issue is not whether or not one trusts teachers, the issue is whether or not those in charge of an organization do their job to make it effective.
Stop blaming teachers. If they fail to be effective, assist them to be so or get rid of them. What exactly is the problem?