In an effort to keep educational costs in check, America's cash-strapped states, local school districts and charter schools are hiring less costly novice teachers. Some of the new hires are energetic college graduates supplied for two-year stints by programs such as Teach for America.

In the late 1980s, most of the nation's teachers had considerable experience -- only 17 percent had taught for five or fewer years. By 2008, however, about 28 percent -- or more than one in four of America's teachers -- had less than five years of experience.

The proportions of novices in the classroom are particularly high in schools in underprivileged areas.

Some observers applaud the rapid "greening" of the teaching force because they think that experienced teachers are not needed. But this view is shortsighted. Although a constant flow of new recruits is healthy, research shows that teacher experience matters in important ways:

  • Experienced teachers, on average, are more effective at raising student achievement. In research I have done with colleagues in my home state of North Carolina, experienced teachers greatly boost student achievement in elementary, middle and high schools alike. This pattern holds even after we adjust for the fact that experienced teachers are more likely to work in schools with more advantaged students.

  • Teachers become more effective as they gain experience. Researchers have long documented that teachers improve dramatically during their first few years on the job. Less clear has been what happens after those early years. In our new research on middle school teachers in North Carolina, we find that math teachers become increasingly effective at raising the student test scores through about 15 years, at which point, they are about twice as effective as novices with two years of experience.

    The productivity gains are less dramatic for middle school English teachers, but follow the same trajectory.

  • Experienced teachers also strengthen education in numerous ways beyond improving test scores. Our research suggests that as middle school teachers gain experience, they become increasingly adept at producing other important results, such as reducing student absences and encouraging students to read for recreational purposes outside of the classroom.

    More experienced teachers often mentor young teachers and help to create and maintain a strong school community.

    Also, as other research has shown, constant teacher turnover is disruptive for schools and harmful to students, especially in disadvantaged schools. All too often, inexperienced teachers are initially assigned to disadvantaged schools, where the challenges of maintaining order and effectively instructing students are very high.

    Those who make the case for hiring novice teachers typically cite the apparent success of many charter schools that employ such teachers and hail the effectiveness of many Teach for America recruits. TFA teachers are often no weaker than other teachers in the heavily disadvantaged schools into which they are placed -- and some may do better at teaching math.

    But Teach for America instructors do not stay around very long. More than half leave after fulfilling their two-year commitment, and more than 80 percent do so after three years. Even strong charter schools have similar difficulties with teacher retention.

    Schools with high teacher turnover must repeatedly spend costly time on recruiting, mentoring, socializing and training newcomers -- only to see the trained teachers move on.

    So the challenge is to attract newly minted teachers with clear long-term potential, give them opportunities to develop their skills and retain the most successful. For that to happen:

  • Salaries must be adequate. Teachers should be able to expect that as they gain experience, their salaries will rise in line with what college graduates earn in comparable professions.

  • Attention must be paid to working conditions. Teachers who experience poor working conditions are more likely to leave a school -- or the profession.

  • Politicians should stop designating teachers as scapegoats. Especially when they work with socially disadvantaged students, teachers must be provided with the institutional supports they need to be effective and to steadily advance their skills.

    Wonderful as it is for bright college graduates to bring new energy and skills to the classroom, schools pay a high price for too much teacher turnover.

    Surely America can do better. The all-important starting point is to recognize the value of teacher experience and then learn how to develop and reward it.

    Helen F. Ladd is a professor of public policy and economics at Duke University's Sanford School of Public Policy and a member of the Scholars Strategy Network.