Jerry D. Weast says he is mindful of the pressures all around him. The federal government requires yearly improvement in his schools' test scores. The County Council insists he trim his budget. Board of Education candidates debate his new grading policy. As for the parents of his 140,000 students, their lists of demands go on and on.

Yet among U.S. school superintendents, Weast is one of the fortunate ones.

He has job security, signed to a second four-year contract in Montgomery County. His reappointment last year marked the first time in 40 years that a Montgomery superintendent who was hired from outside the county will get to serve beyond one term.

The average tenure of an urban or suburban superintendent in the United States has dropped below six years, and the Washington region alone is facing four vacancies -- in the District, Falls Church and Fairfax and Howard counties.

With pressure to improve student achievement under the No Child Left Behind law, Weast and other local school leaders predict even more turnover. "Superintendents are responsible for leading change. So when tensions escalate, the superintendent is the easiest link to eliminate, hence short tenures," said Jack D. Dale, who is in his eighth year leading Frederick County schools.

The current issue of a magazine published by the American Association of School Administrators reports on the tenure-shortening factors -- financial constraints, divided school boards, teachers unafraid to speak their minds, ethical and legal entanglements. It also documents two instances in which superintendents under pressure committed suicide.

Weast said he can gauge the climate by talking to job-search firms. "School systems all over the country are looking for people who have proven themselves over time in very difficult situations," he said.

In the Washington area, two of four upcoming vacancies appear related to political clashes. Former Howard County superintendent John O'Rourke, whose last day was yesterday, resigned after the school board did not renew his contract and criticized his communication skills. Paul L. Vance resigned as D.C. superintendent in November, saying he did not "want to be bothered with" such issues as budget problems and Mayor Anthony A. Williams's attempt to take control of the city's schools.

Superintendents Daniel A. Domenech of Fairfax County and Mary Ellen Shaw of Falls Church both announced retirements recently, following what their boards called very successful tenures. The last day for Domenech, who will go to work for a New York textbook publisher, will be Friday, and Shaw's retirement is effective at the end of the school year.

In the future, several superintendents said, it is going to be more difficult to claim success. The No Child Left Behind law, passed by Congress in 2002, puts a "needs improvement" label on any school that has not shown steady progress in test scores. "Things will get worse," said Domenech, an outspoken critic of the federal law. "No Child Left Behind will eventually have every school system in the country failing. Many superintendents will lose their jobs over that."

Even Washington area districts whose schools have performed well above the national average on standardized tests often do not keep superintendents long, particularly if the administrator in question was hired from outside the district. This is a natural consequence of elected school boards acquiring new members who weren't involved in hiring the superintendent, the administrators say.

"A superintendent finds himself or herself moving forward with the agenda hired for, and the agenda is changed in midstream by new school board members," said Jean S. Murray, superintendent of Stafford County schools.

"Another issue . . . is the idea that one person can come in and 'fix' a school [system] that has become 'broken' over a long period of time," Murray said. "A superintendent needs time, board and community support and resources to move an ailing system in a positive direction."

Alexandria school board member Sally Ann Baynard, who teaches political science at Georgetown University, joked that she would do harm to anyone who tried to steal her city's superintendent, Rebecca L. Perry. But Baynard said she sees a persistent structural problem in most school board-superintendent relationships.

"We who are elected to the boards usually come to the job of policymaking for a school system with no experience in educational policy and very little experience in policymaking in general," she said, yet "are flush with the success of having run an election and have all sorts of ideas." The superintendent those board members have to deal with, in many instances, has been working with education policy for years and might see his or her new bosses as interfering amateurs, Baynard said.

Thomas DeBolt, superintendent of Manassas Park schools, said he believes this situation has worsened in the past decade in Virginia as more districts took advantage of a state law that allowed elected, rather than appointed, school boards. "Too often board members pursue a specific political agenda rather than attempting to improve the overall school system," he said.

Domenech agreed. "Board members are more political today than ever," he said. "Superintendents must be very adept at dealing with politics if they are to survive."

Just doing their jobs, superintendents say, makes them easy targets for criticism because they must make decisions that will offend some powerful group involved in schools, such as teachers or parents or taxpayers. Courtney Watson, chairman of the Howard County school board that decided not to renew O'Rourke's contract, said, "I work in private industry, and I think a superintendent is similar to being the CEO of a large company."

Dale Sander, superintendent of Fredericksburg city schools, said a superintendent in a large system often works 12- to 14-hour days. And although the salaries are high by government standards -- often more than $200,000 a year in large districts -- "superintendents find they can often do better in the private sector," he said.

Watson said another reason for shorter superintendent tenures might be that there is strong demand for good administrators. Superintendents who have shown some success are more likely to get attractive offers from other districts and thus switch jobs, she said.

Superintendents in the Washington area said they are not sure there is a cure for the growing pressures of their jobs. Domenech suggested more training in politics and public relations. DeBolt said more training for school board members also might help.

Frederick County's Dale said it is going to take much more. He called for a national dialogue on school organization.

"Until organizational leadership, ownership and turf issues are resolved, superintendents will have increasingly shorter tenures, especially in systems begging for significantly different results for their children," he said.