GREENSBORO, N.C. — The retirement of thousands of baby boomer teachers coupled with the departure of younger teachers frustrated by the stress of working in low-performing schools is fueling a crisis in teacher turnover that is costing school districts substantial amounts of money as they scramble to fill their ranks for the fall term.

Superintendents and recruiters across the nation say the challenge of putting a qualified teacher in every classroom is heightened in subjects like math and science and is a particular struggle in high-poverty schools, where the turnover is highest. Thousands of classes in such schools have opened with substitute teachers in recent years.

Here in Guilford County, N.C., turnover had become so severe in some high-poverty schools that principals were hiring new teachers for nearly every class, every term. To staff its neediest schools before classes start on Aug. 28, recruiters have been advertising nationwide, organizing teacher fairs and offering one of the nation’s largest recruitment bonuses, $10,000 to instructors who sign up to teach Algebra I.

“We had schools where we didn’t have a single certified math teacher,” said Terry Grier, the schools superintendent. “We needed an incentive, because we couldn’t convince teachers to go to these schools without one.”

Guilford County, which has 116 schools, is far from the only district to take this route as school systems compete to fill their ranks. Kate Walsh, president of the National Council on Teacher Quality, a nonprofit policy group that seeks to encourage better teaching, said hundreds of districts were offering recruitment incentives this summer.

Officials in New York, which has the nation’s largest school system, said they had recruited about 5,000 new teachers by mid-August, attracting those certified in math, science and special education with a housing incentive that can include $5,000 for a down payment.

New York also offers subsidies through its teaching fellows program, which recruits midcareer professionals from fields like health care, law and finance. The money helps defer the cost of study for a master’s degree. The city expects to hire at least 1,300 additional teachers before school begins on Sept. 4, said Vicki Bernstein, director of teacher recruitment.

Los Angeles has offered teachers signing with low-performing schools a $5,000 bonus. The district, the second-largest in the country, had hired only about 500 of the 2,500 teachers it needed by Aug. 15 but hoped to begin classes fully staffed, said Deborah Ignagni, chief of teacher recruitment.

In Kansas, Alexa Posny, the state’s education commissioner, said the schools had been working to fill “the largest number of vacancies” the state had ever faced. This is partly because of baby boomer retirements and partly because districts in Texas and elsewhere were offering recruitment bonuses and housing allowances, luring Kansas teachers away.

“This is an acute problem that is becoming a crisis,” Ms. Posny said.

In June, the National Commission on Teaching and America’s Future, a nonprofit group that seeks to increase the retention of quality teachers, estimated from a survey of several districts that teacher turnover was costing the nation’s districts some $7 billion annually for recruiting, hiring and training.

Demographers agree that education is one of the fields hardest hit by the departure of hundreds of thousands of baby boomers from the work force, particularly because a slowdown in hiring in the 1980s and 1990s raised the average age of the teaching profession. Still, they debate how serious the attrition will turn out to be.

In New York, the wave of such retirements crested in the early years of this decade as teachers left well before they hit their 60s, without a disruptive teacher shortage, Ms. Bernstein said.

In other parts of the country, the retirement bulge is still approaching, because pension policies vary among states, said Michael Podgursky, an economist at the University of Missouri. California is projecting that it will need 100,000 new teachers over the next decade from the retirement of the baby boomers alone.

Some educators say it is the confluence of such retirements with the departure of disillusioned young teachers that is creating the challenge. In addition, higher salaries in the business world and more opportunities for women are drawing away from the field recruits who might in another era have proved to be talented teachers with strong academic backgrounds.

“The problem is not mainly with retirement,” said Thomas G. Carroll, the president of the National Commission on Teaching and America’s Future. “Our teacher preparation system can accommodate the retirement rate. The problem is that our schools are like a bucket with holes in the bottom, and we keep pouring in teachers.”

The commission has calculated that these days nearly a third of all new teachers leave the profession after just three years, and that after five years almost half are gone — a higher turnover rate than in the past.

All the coming and going of young teachers is tremendously disruptive, especially to schools in poor neighborhoods where teacher turnover is highest and students’ needs are greatest.

According to the most recent Department of Education statistics available, about 269,000 of the nation’s 3.2 million public school teachers, or 8.4 percent, quit the field in the 2003-4 school year. Thirty percent of them retired, and 56 percent said they left to pursue another career or because they were dissatisfied.

The federal No Child Left Behind law requires schools and districts to put a qualified teacher in every classroom. The law has led districts to focus more seriously on staffing its low-performing schools, educators said, but it does not appear to have helped persuade veteran teachers to continue their service in them.

Tim Daly, president of the New Teacher Project, a group that helps urban districts recruit teachers, said attrition often resulted from chaotic hiring practices, because novice teachers are often assigned at the last moment to positions for which they have not even interviewed. Later, overwhelmed by classroom stress, many leave the field.

Chicago and New York are districts that have invested heavily and worked with teachers unions in recent years to improve hiring and transfer policies, Mr. Daly said.

“But most of the urban districts have no coherent hiring strategy,” he said. Many receive thousands of teacher applications in the spring but leave them unprocessed until principals return from August vacations, when more organized suburban districts have already hired the most-qualified teachers, he said.

“There isn’t any maliciousness in this,” Mr. Daly said, “it’s just a conspiracy of dysfunction.”

In Guilford County, Washington Elementary School, which serves students from a housing project, had churned through several principals and most of its teachers several years ago, and had repeatedly failed to make federal testing goals, said Dr. Grier, the superintendent.

“Teachers were worried it was becoming a failing school,” Dr. Grier said. To rebuild morale, he recruited a principal from Chicago, Grenita Lathan. Her first year at Washington was a nightmare, Ms. Lathan said, because her predecessors had been so panicked to fill classroom vacancies that they had hired “just anybody.”

“All they wanted was warm bodies in the classroom,” she said. At job fairs, qualified teachers she tried to hire shunned her, she said.

Under Guilford County’s incentive program, math or reading teachers who sign on at any of 29 high-poverty schools receive bonuses of $2,500 to $10,000. They can earn additional bonuses if they raise achievement.

Those incentives helped Ms. Lathan recruit solid teachers last year, she said, and after much tutoring and hard work, students met federal testing targets. This summer all but one teacher signed up for another year.

Other Guilford County schools have also used the incentives to hire promising people.

Rebecca Rheinheimer moved from Indiana this summer, attracted by a $2,500 bonus to teach at Oak Hill Elementary, where the teaching staff has been strengthened by the use of such bonuses. The school, in High Point, met its federal testing targets this spring for the first time in several years.

Margaret Eaddy-Busch, a veteran math teacher, moved from Philadelphia this summer to teach at Dudley High, which had become known as a hard-to-staff school. She will receive a $10,000 bonus for teaching Algebra I.

“If I survived in Philly for 10 years,” Ms. Eaddy-Busch said, “I’ll do just fine here.”

But it remains unclear whether the incentive program will retain good teachers as effectively as it attracts them.

“It’s challenging to teach in these high-needs schools,” said Mark Jewell, president of the local teachers union. “These new teachers will have a trial by fire, and then it’ll be a revolving door.”