Updated

More teachers, it seems, are ready to leave their schools behind.

Forty percent of public school teachers plan to exit the profession within five years, the highest rate since at least 1990, according to a study being released Thursday.

The rate is expected to be even greater among high school teachers, half of whom plan to be out of teaching by 2010, according to the National Center for Education Information.

Retirement is the dominant factor, as the public teaching corps is aging fast, say surveys of teachers in kindergarten through grade 12.

In 1996, 24 percent of teachers were age 50 or older. Now, in 2005, 42 percent of teachers are.

"I'm ready to do what I want to do," said Pat Jeppe, 59, a middle school teacher in Southaven, Miss., who plans to retire in a couple of years after teaching for the past 35. "I finally have grandchildren and I want to be with them and go to their school functions," Jeppe said.

The projected turnover rate will deprive school districts of an enormous amount of teaching experience just as the U.S. pushes to get a top instructor in every class.

The proportion of teachers with at least 25 years in the classroom has more than doubled in the past 15 years, from 12 percent to 27 percent.

The teaching corps has grown older across the board because more people are moving into the field in their 30s and 40s, said Emily Feistritzer, president of the center, a private organization that specializes in survey research of school trends.

"We're going to have tremendous turnover, but I happen to think it's a tremendous opportunity rather than a hand-wringing time," she told The Associated Press.

"We'll have 40 percent of the teaching force replaced by mid-career switchers and people with life experience, people with altruistic motives for coming into teaching," Feistritzer said.

In 1990, 74 percent of teachers predicted they would still be in the classroom five years later. In the surveys, that total dropped to 66 percent in 1996 and 60 percent this year.

School districts are expanding their recruitment beyond colleges of education to other career fields, where experts in math, science and other subjects are being trained to teach.

Broadening this pool of prospective teachers will help fill the void of retiring teachers, said Michelle Rhee, president of The New Teacher Project, a nonprofit group that helps some of the largest school districts recruit teachers.

Younger people remain a big force in public teaching, with one in three teachers 39 or younger. But many of those teachers no longer think of teaching as a 30-year career.

"There is a growing realization that the mind-set is shifting, that they don't consider teaching to be a lifelong profession," Rhee said.

Maia Sheppard is one of those people. She worked for three years in a New York City public school, where she taught global history, wrote curriculum and helped immigrant students try to learn English and meet state standards in several other subjects.

After three years, she had enough and quit teaching.

"It was just all the time, nonstop," she said. "I just didn't have time to do anything else but school."

Now 32 and living in Minneapolis, Minn., Sheppard is pursuing a doctorate in education.

Overall, 83 percent of teachers say they are satisfied with their jobs, a level that has held steady over the last 15 years. Yet, beyond retirement, teachers say they have plenty of reasons to consider leaving: concerns over pay, dissatisfaction with school bureaucracy or plans to work in another education job, among other factors.

The survey of 1,028 public school teachers, taken in March through June, has a margin of sampling error of plus or minus 3 percentage points.