Joshua Starr’s resignation after three and a half years as superintendent of Montgomery County Public Schools means his tenure will have been shorter than the national average, which is close to six years, according to AASA, a national association of school superintendents.

But Starr’s tenure is typical for the nation’s large urban school systems. The Council of the Great City Schools, an organization of more than 50 of the county’s large urban school systems, found that the average tenure for superintendents fell from 3.6 years in 2010 to 3.2 years in 2014.

“It’s very difficult for the large systems, whether they’re large urban systems or large suburban systems, to retain superintendents over the long haul,” said James Harvey, executive director of the National Superintendents Roundtable, a coalition of about 100 superintendents.

The rapid turnover is driven in part by local politics, as school board candidates often campaign on a platform of pushing out the sitting superintendent or taking the schools in a new direction, Harvey said. Expectations of superintendents have risen quickly during the past decade: At the same time as the federal No Child Left Behind law has required that schools close persistent achievement gaps, the number of low-income and immigrant students has risen, and many states have cut funding for public schools.

“It’s a much more difficult job today than it was even 10 years ago,” Harvey said. “The demand is for superintendents to produce superior results for all students at a time when the intake for schools is much more challenging.”

The superintendent churn gets less attention than turnover among teachers and principals, but it presents a real challenge, according to education observers and policy wonks.

“Communities that are trying seriously to turn around their schools see this as a 10- to 20-year iterative process, where they have bumps on the way, they make changes on the way … that’s what big, long-term sustainable change looks like,” said Elaine Weiss, a researcher at the Economic Policy Institute and Montgomery County parent who is supportive of Starr. “If you’re out in three and a half years, you have barely made a dent.”

There’s no consensus that superintendents can be credited or blamed for student achievement in their districts. Researchers at the Brookings Institution concluded in 2014, after examining districts in North Carolina and Florida, that superintendents account for less than one percent of student achievement and that student achievement does not improve with superintendents’ longevity.

“Superintendents whose tenure is associated with sizable, statistically reliable changes in student achievement in the district in which they serve, controlling for the many other factors that affect student achievement, are quite rare,” the researchers wrote.

School systems in the Washington area have experienced both revolving doors and stability at the top, but many local systems now have superintendents who have been in the job only a short time.

Former Loudoun County Superintendent Edgar B. Hatrick III retired in 2014 after 23 years, and Prince William County Superintendent Steven L. Walts has served for 10 years. Karen Garza has only led Fairfax County for 1.5 years, but before she arrived in 2013, Dan Domenech and Jack Dale served six years and nine years, respectively.

At the other end of the spectrum, Prince George’s County has seen eight superintendents in the past 16 years.

With Starr’s departure, Montgomery County enters a competitive market for big-system superintendents. There are more than a dozen major superintendent positions open, including in North Carolina’s Charlotte-Mecklenburg district and in Nashville, Tenn.

The school system in Hillsborough County, Fla., — one of the largest school districts in the country — also has a vacancy, after the school board voted last month to fire well-liked Superintendent MaryEllen Elia, who was not only Florida’s Superintendent of the Year but a finalist for National Superintendent of the Year. In her decade on the job, Elia had succeeded in helping raise the achievement of low-income students and had won praise from business leaders, fellow superintendents and her local teachers union. In deciding to fire her, the school board cited concerns about racial disparities and special education and said Elia should have worked more closely with the community.

Such leaders can be difficult to find, and superintendents often move between districts.

“I think there are only a limited number of people with the skill set required to lead a very large district successfully,” Harvey said, adding that a successful superintendent has to be an educator, an administrator and a politician. “There aren’t very many men and women who have all of that experience to draw on.”

Bill Turque contributed to this report.

Correction: An earlier version of this story inaccurately described the tenure of former Fairfax County superintendents Dan Domenech and Jack Dale. The story has been  corrected.