Fully half of all teachers nationally leave the profession within the first five years, with special retention challenges in urban school systems.

New research out Wednesday shows that the problem is especially acute in the Philadelphia School District, where over seven years, an average of 27 percent of teachers left their schools in a given year — with some transferring to other schools — and about 15 percent left the school system entirely, according to the analysis by the Philadelphia Education Research Consortium.

Philadelphia's teacher mobility numbers were higher than national averages. Across the nation, 18 percent of urban teachers left their schools per year. Among schools where more than three-quarters of students live below the poverty line, the teacher mobility rate is 22 percent, higher than the 14 percent of all other schools.

The practical implications are real: Teacher mobility has a negative effect on student achievement. It means monetary costs — hiring new teachers is expensive — and it also often leads to inequalities within systems, concentrating inexperienced teachers at lower-achieving, tougher-to-staff schools.

"Teachers are the most important influence in schools on student achievement, which makes attracting and retaining excellent teachers a high priority for all school districts," wrote the researchers, including Matthew P. Steinberg, an assistant professor of education at the University of Pennsylvania's Graduate School of Education, and Ruth Curran Neild, director of the research consortium.

The researchers examined data from the 2009-10 through 2015-16 school years, which encompassed a period of upheaval for the system, with school closings, layoffs, school reconstitutions, and a prolonged teacher contract standoff. (The report does not delve into reasons why Philadelphia teachers chose to leave their schools.)

Among the key findings: Relatively few teachers left the district to work in charters (3 percent) or other Pennsylvania districts (2 percent). Those who did leave most often joined Aspira of Pennsylvania or Mastery charter schools or the Upper Darby or North Penn school districts.

Those relatively low numbers were particularly surprising to Neild, who has long examined Philadelphia educational trends.

"For years in Philadelphia, there's been this narrative that Philadelphia public schools serve as the farm team for surrounding districts," Neild said. But that clearly turned out not to be the case, based on the data.

Those teachers most likely to leave were those with the least and most experience — more than half of all first-year teachers moved schools or left the district after a year, and once teachers hit the 30-year mark, mobility rates spiked.

The highest rates of mobility were seen among math, science, and English teachers.

And, the researchers found, the typical district teacher who left his or her school but continued to teach did so the next year in a school that was whiter and wealthier, with higher student achievement.

A dwindling supply of new teachers complicates the problem, too. Pennsylvania used to license more than 14,000 new teachers annually. Now, it issues certificates to fewer than 5,000.

For the district, "the high turnover among early-career teachers, though not unique to Philadelphia, is an area for ongoing attention," the researchers wrote. "Some of this mobility may be normal and beneficial as teachers seek school assignments that best match their skills. But some of this mobility may be the result of receiving difficult teaching assignments without enough support."

Lee Whack, a School District spokesperson, stressed that the district was not alone in its issue with early-career teacher turnover. But, Whack said, new efforts have been made to keep teachers from leaving, including a strengthened new-teacher orientation and more teacher coaching.

"We are making every effort to ensure that our classroom leaders are equipped to help our students learn and grow, and that we reduce teacher mobility when appropriate," Whack said.