In New York City public schools, even the principals are unionized. After five years school principals receive tenure and can only be fired by the superintendent after a three-year review process.

Consequently, it is close to impossible to fire them for failing to improve students' performance.

• A majority of New York City's 1,000 principals are mediocre, and as many as 25 percent are incompetent, writes former chancellor Joseph Fernandez in Tales Out of School.

• There were 21 documented cases in the past year where principals of schools in which reading scores had dropped sharply were granted tenure.

• Just under a third of elementary school principals up for tenure next year are running schools where the number of third-graders reading above remedial levels declined 15 percent, according to the New York Times.

• And only last year was state law changed to allow superintendents to fire principals not just for criminal acts, but "persistent educational failures."

Only 22 of the city's 500 or so tenured principals have ever faced charges -- and mostly for criminal acts or gross negligence; not one has been challenged for educational failures. Usually, bad principals are just transferred to a different school.