Editor's note: William J. Bennett, a CNN contributor, is the author of the newly published "The Book of Man: Readings on the Path to Manhood." Bennett, the Washington fellow of the Claremont Institute, was U.S. secretary of education from 1985 to 1988 and was director of the Office of National Drug Control Policy under President George H.W. Bush.
(CNN) -- A good teacher not only improves a child's test scores in the classroom, but also enhances his or her chances to attend college, earn more money and avoid teen pregnancy, according to a new seminal study.
The study, conducted by economists Raj Chetty and John N. Friedman of Harvard and Jonah E. Rockoff of Columbia, tracked 2.5 million students over 20 years from a large urban school district from fouth grade to adulthood, making it one of the largest and most consequential educational studies in recent years.
Their findings focus on the long-term impact of teachers based on their "value-added" ratings. This refers to the average test-score improvement for a teacher's students, adjusted for differences across schools and classrooms such as prior test scores. Evaluating teachers based on student performance has been the subject of much debate among teachers, unions and policymakers.
Teachers and teachers' unions have been largely critical of the value-added approach, arguing that test scores are not good indicators of teacher quality. However, many reformers, like Stanford's Eric Hanushek, argue that value-added ratings are some of the most accurate indicators for evaluating teachers and improving student performance.
This study puts the value-added approach to the test in a new way, measuring the short-term impact in the classroom as well as long-term impact, like a student's collegiate, career and family success. The authors found that "when a high VA teacher joins a school, test scores rise immediately in the grade taught by that teacher; when a high VA teacher leaves, test scores fall."
Most importantly, the study discovered that "students assigned to higher VA teachers are more successful in many dimensions. They are more likely to attend college, earn higher salaries, live in better neighborhoods and save more for retirement. They are also less likely to have children as teenagers."
The effect of a good teacher on a child's life is monumental. In financial terms, the study notes that replacing a teacher whose true value-added is in the bottom 5% with a teacher of average quality would generate lifetime earnings gains worth more than $250,000 for the average classroom.
On the other hand, "If you leave a low value-added teacher in your school for 10 years, rather than replacing him with an average teacher, you are hypothetically talking about $2.5 million in lost income," said Friedman.
The authors, two of whom were skeptical at first of the merits of value-added metrics, conclude the study by recommending that schools adapt value-added measures and fire the lowest-performing teachers.
Given the enormous contribution of good teachers to the lives of students, one would think the organizations that represent teachers would hail this study and disseminate its findings. This is not so.
Predictably, the teachers' unions have been silent. Acknowledging the realities of this study would require the unions to make distinctions between good and bad teachers and hold them accountable for their performance, something they seem incapable of practicing, much less institutionalizing.
It is ironic that a report that praises great teachers seems to be ignored by teacher organizations.
This study shows that, second only to parents, teachers are the most important part of a child's education. Great teachers make a great difference; poor teachers hurt a child's life chances. Isn't that all we need to know to embark upon a serious effort to reward good teachers and encourage poor teachers out of the profession?
The opinions expressed in this commentary are solely those of William J. Bennett