A stable teaching staff is crucial to a school’s academic success, but we found that thousands of the city’s most vulnerable children attend Philadelphia schools where teachers shuffle in and out at destabilizing rates — 25 percent is cause for alarm, experts say.

Our investigation found 26 district schools that have lost at least 25 percent of their teachers for four years straight or lost more than one-third in each of the last two school years. Richard M. Ingersoll, a University of Pennsylvania professor and expert in school staffing, called these findings “appalling.”

Why is there so much teacher churn?

Schools beset by turnover typically fill their openings with teachers new to the district. More than half of the teachers who currently work at the 26 schools have less than four years of experience in the system. At Jay Cooke Elementary School, for example, which has a faculty of 30, more than 130 teachers have worked there in just seven years.

Many new teachers leave because they feel they don’t get adequate coaching and support. Then they’re replaced by recruits just like them, and the cycle begins again.

Schools with staff churn also get teachers who were pushed out of their old assignments through the “force transfer” process, which gives principals the power to remove teachers for misconduct or poor performance, or because of changing enrollment. But it also guarantees those teachers jobs in a different school, which often ends up being those with high turnover. Sometimes, teachers choose to leave schools because of poor leadership, with some teachers saying principals who fail to keep order or motivate them can drive them away.

What is the district doing to address leadership and turnover issues?

Superintendent William R. Hite Jr. spelled out the district’s belief in strong leadership in an “action plan” that pledged, in part, to fill 100 percent of schools with “great” principals. Deputy chief talent officer Terri Rita said the school system is increasingly focused on principal quality. However, teachers say concern about leadership is still the number-one reason they quit, according to an exit survey of departing employees.

District officials also said that they are working to aggressively recruit teachers to work in hard-to-staff schools, and that they have revamped their human resources department, assigning “talent partners” to work directly with principals to help find top recruits. They say their work is hampered by a national shortage of certified teachers.

What other forces drive teacher turnover?

Teachers often say they feel they don’t have enough resources to succeed. This makes sense. The overwhelming majority of district students live in poverty, and the gap between funding for wealthy and poor districts in Pennsylvania is larger than anywhere else in the country, according to an analysis of federal education data. A lawsuit before the state Supreme Court alleges that the way schools are funded in the commonwealth is illegal and demands a judicial remedy on behalf of districts like Philadelphia. Plus, the district is still recovering from a 2013 financial crisis that forced it to close a raft of schools and lay off hundreds of teachers.

One consequence is that schools don’t have as much support staff as they used to. Some teachers who leave say they’re abandoning the profession. This is especially common among inexperienced teachers. Others who leave complain about the sorts of toxic building conditions that Inquirer reporters exposed last year in its Toxic City: Sick Schools series. The fifth most common reason teachers leave is pay. Salaries start at $45,000 and average about $70,000, far below those in neighboring Cheltenham, which start about the same but average $90,000.

What can be done to reduce teacher turnover?

Some Philadelphia schools in challenged neighborhoods have succeeded in keeping a stable teaching force long-term. Teachers at these schools say they feel supported by one another and by school leaders, so they stay, often producing academic gains for students.

Districts such as Dallas and New York are offering significant bonuses to teachers who agree to work in hard-to-staff schools.

In Dallas, highly rated teachers can earn up to $15,500 extra a year for three years when they agree to work in a struggling school. The bonus program helped one Dallas school win its first “pass” rating from the state in more than a decade. The state’s governor is now pushing the state Legislature to replicate the program in other districts.

New York’s bonus program kicked off in November. It allows teachers to play a role in how their schools are run, a feature that union leaders say is as appealing to its members as the extra income.

When the Philadelphia district settled on a contract with the teachers’ union several years ago, district leaders had proposed offering bonuses to attract teachers to hard-to-staff schools. To pay for it, they sought to use money allocated for teachers who earn advanced degrees. But the union opposed the plan. Both sides remain skeptical of bonus pay.

Does paying teachers more for having advanced degrees boost academic performance?

Not really, researchers say. Studies published in 2003 and 2004 found a positive link between teachers with master’s degrees and higher elementary math results. But more recent research is less clear whether teachers with graduate degrees lead to students scoring higher in reading or math.

Will the district and union address the force transfer process, bonus pay, or other tactics to tackle teacher turnover?

Changes would need to be made at the bargaining table. The district’s current contract with the Philadelphia Federation of Teachers runs through Aug. 31, 2020. Negotiations on terms of a new contract will likely start later this year.