On Thursday, a million New York City children will return to school. Educators have long been concerned about a “summer slide” — the learning loss that often occurs when students are out of school for two months. It’s a serious problem. But it’s not just students who can slide backward during these months. Facing political and budgetary pressures, an entire school system can slide without strong leadership. That’s now happening in New York.

In July, two weeks after the State Legislature reauthorized mayoral control of the public school system, Mayor Bill de Blasio’s administration quietly announced a policy reversal: In the coming year, schools will once again be forced to hire teachers that no other school has wanted to hire. As a former principal of a high school in the Bronx, I find it hard to imagine receiving worse news.

The new policy concerns the approximately 800 teachers in the city’s Absent Teacher Reserve pool, a remnant of a teacher-placement system based on seniority, not what’s best for schools or children. These are teachers who, for whatever reason, have not gotten a job in any of the city’s 1,700 schools, sometimes for many years. The city is in this position because the union contract makes dismissing teachers a virtual impossibility. A result is that taxpayers spend more than $150 million a year to pay them not to teach. Given the alternative, though, it’s money well spent.

Data recently released by the city reveals that a third of teachers in the reserve pool had faced disciplinary or legal charges, and as has been reported nearly 20 percent received poor ratings — startling given that 93 percent of New York City teachers are rated effective or better. Teachers in the reserve pool are supposed to actively search for new jobs within the system, but a 2014 analysis shows more than half had not applied for a single position in the previous year. That suggests a level of complacency that would be unthinkable among great teachers, who spend nights and weekends finding new ways to improve their practice and help their students learn.