American Federation of Teachers President Randi Weingarten is promoting a terrific idea -- a rigorous national exam that teachers must pass before they are fully qualified to lead a class.

"We must do away with the common rite of passage whereby new teachers are thrown into classrooms, expected to figure things out, and left to see if they (and their students) sink or swim," she wrote in a recent Wall Street Journal commentary.

"Such a haphazard approach to the complex and crucial enterprise of educating children is wholly inadequate. It's unfair to both students and teachers, who want and need to be well-prepared to teach from their first day on the job."

Teachers and students across the country would benefit from a tough exam that tested a teacher's mastery of subjects and his or her ability to teach those subjects. Such an exam could require would-be teachers, before they are licensed, to demonstrate the skills and understanding they'll need to be effective.

As Weingarten suggests, the process could be modeled after the bar exam for lawyers or the board certification of doctors.

Passing a nationally recognized teachers' exam would become a badge of honor and a sign of competence that would reassure parents, students and other teachers. That is vital. "Unless the teaching profession limits entry to those who are fully equipped to succeed in the classroom, it will never be held in the same high esteem as medicine or law," says Arthur McKee of the National Council on Teacher Quality.

Weingarten should find plenty of allies in this effort.

In too many states, teacher certification tests are so easy that "there is virtually no reason to administer them, because almost everyone passes," McKee told us.

The best teachers are magicians. They have the capacity to inspire, to keep a young person focused, to develop the mind and, perhaps above all, to motivate.

Many people have neither the talent nor the drive to provide that leadership in the classroom -- and fortunately they follow other pursuits.

But too many ill-equipped people do enter the teaching profession. That doesn't help anyone.

Good idea, Ms. Weingarten: A national bar exam for teachers.