The goal for Montgomery County’s school board? Find an ambitious but budget-conscious superintendent who can thrive over the long haul.

The odds of finding such a candidate? Long, but not impossible.

Of the 25 largest school districts in the country, fewer than half have schools chiefs that have been there longer than three years. Only three have superintendents who have lasted longer than 10 years. And only one has a longer tenure than current Montgomery County superintendent Jerry D. Weast, who was hired 12 years ago.

“We don’t want to go in with an unrealistic expectation,’’ said School Board President Christopher Barclay. “We offer four-year contracts, and we hope the person would be able to commit to that, but we know the reality that superintendents stay much shorter.’’

Research has linked student success with a superintendent's staying power. Yet it’s difficult for districts to keep superintendents for more than a few years. The chief must be academically innovative and politically astute, able to win over unions, school boards and parents.

“This is one of most volatile positions in civic service,’’ said Daniel A. Domenech, executive director of the American Association of School Administrators and a former Fairfax County superintendent. “It’s part in parcel because it’s a very complex job, subject to politics and to public pressure.’’

It’s not necessarily that superintendents want to go. According to Domenech, there have long been tensions between the long-term visions of schools chiefs and the school boards that typically hire and fire them. Their attitudes often change with each election cycle.

In the past 10 years, Domenech said, being a successful superintendent has become more difficult. Education has become an even more high-stakes game, with aggressive federal initiatives such as Race to the Top and No Child Left behind. The constant demand for evaluation leaves little room for error. If the results aren’t immediately good, it’s not unusual for a superintendent to be sent packing.

Schools chiefs have adapted. They market themselves as “change agents” along the lines of former D.C. chancellor Michelle Rhee, who championed ambitious agendas for quickly boosting test scores but resigned after 3 1/2 years when her patron, Mayor Adrian Fenty, lost his re-election bid.

Education experts say there is little doubt that students are better off if an administrator is able to set long-term goals for classrooms. They look to the Broad Prize, an annual$1 million award given to the best performing urban school district, as an example.

Since the award’s inception in 2002, only two winners had recent superintendents who served for fewer than five years. Last year, Montgomery was a finalist but was beaten out by Gwinnett County Public Schools in Georgia — where J. Alvin Wilbanks has overseen the system for 15 years. He is the only superintendent, among the 25 largest in the country, whose tenure has lasted longer than Weast’s.

An analysis of research on school leadership by Mid-continent Research for Education and Learning, a Denver-based nonprofit, revealed that students typically perform better when their school chiefs stay in office longer.

Because the interview process is done behind closed doors, it is not easy to tell whether Montgomery’s school district will be an example of, or an exception to, this trend.

The job certainly has appeal: It’s a high-profile post in a high-performance district. But Weast’s success might intimidate. He boasts about his cozy relationship with unions, his easy rapport with the school board and his popularity among principals. No stranger to picking fights with the county government or state superintendents, Weast’s demonstrated commitment has helped to prevent irreparable damage to relationships, according to a local union representative.

“It has to do with the fact that he’s been around for a long time,’’ said Doug Prouty, president of the Montgomery County Education Association. “When there are difficult times, you know he’ll be there to work together with you.’’

Patricia O’Neill is the only current member of the board who was involved in hiring Weast. She remembers the board taking an instant liking to his zest for reducing gaps in racial and ethnic performance in a county that was seeing higher rates of poverty and a more diverse student population.

She also recognizes that his successor won’t be so lucky. The new superintendent will inherit a school system that is high on expectations but with fewer resources than in the past.

“That financial experience is going to be more and more important,’’ O’Neill said. “But I’ve become a bit more skeptical about flashy individuals who can deliver quick results. We don’t want to be a stepping stone for someone’s career. We want someone that has visions of being with us a long time.’’

With retirement near, Weast still keeps a large bell in his office that his mother used to ring when she taught at a one-room schoolhouse in Kansas. But beside it is a new iPad, on which he can easily pull up charts measuring the test performance of “Jerry’s Kids.’’

They are the students who were just starting out when Weast was hired and will graduate within a year of his retirement. The statistics show the achievement gap decreasing, while an increasing number of black and Hispanic students are taking Advanced Placement exams.

“I started in education 42 years ago and it wasn’t unusual then for a superintendent to take 15 or 20 years, because that's the amount of time you needed to get the work done,’’ Weast said. “One year does not make any effect, two years hardly gets you a dot, three only gets you an early trend line.’’