Schools that still have vacancies by October will be sent staffers from the city’s Absent Teacher Reserve, a move that may shrink the costly pool but could also rankle principals.

The policy, first reported by the New York Daily News and confirmed by the education department Monday, marks the city’s latest attempt to reach its goal of cutting the pool in half from its current 822 teachers.

The Absent Teacher Reserve is a group of teachers collecting salaries and benefits without holding full-time positions. Teachers can be placed into the ATR either because their jobs were eliminated or for disciplinary reasons.

Under the new policy, principals have until around October 15 — six months from when hiring begins — to fill their vacancies. After that, city officials say they will make placements from the ATR, even potentially over principals’ objections.

“We will work to find the right fit, and hear and work through concerns that they might have,” education department spokesman Will Mantell said. “But ultimately, we do have discretion to place an educator in a vacancy that exists, and it kind of makes sense.”

The placements will be for one year, rather than a monthly rotation. Mantell said that would allow teachers to participate in training and receive guidance from principals. Teachers who score “Highly Effective” or “Effective” on the observation portion of their evaluation when there is a remaining vacancy will be permanently hired.

Schools Chancellor Carmen Fariña promised in 2014 that she would not endorse “forced placement of staff” as a strategy for shrinking the pool. Though the new policy may require principals to take on teachers, Mantell said it is not an example of forced placement because it only applied to vacancies and will not allow ATR teachers to bump existing teachers from their jobs.

Still, the change could prove unpopular with principals. Under former Mayor Michael Bloomberg, principals were given more power to run their schools and make hiring decisions. The de Blasio administration has, to a certain extent, reined in this power — which has drawn some criticism.

The ATR pool swelled under former Mayor Michael Bloomberg, who aggressively closed struggling schools, and cost the city an estimated $105 million in 2013. Current Mayor Bill de Blasio has pledged to shrink the pool in half.

Measuring the ATR pool can be tricky, since it represents only a snapshot in time and fluctuates throughout the year. Still, city officials argue that, in the aggregate, it has steadily decreased under de Blasio.

The city has undertaken a number of initiatives toward that end, including hiring the former principal of Brooklyn Technical High School to lead efforts to shrink the pool, offering $50,000 severance payments and subsidizing the salaries of teachers hired from the ATR.

Still, at the end of the 2016-17 school year, 822 teachers remained in the pool, according to numbers provided by the education department. This new policy will mark a more aggressive approach to reducing that number. In addition to the placements, teachers in the pool can now be hired across school district lines within their borough.

In an emailed statement, United Federation of Teachers President Michael Mulgrew praised the plan.

“These changes reflect the UFT’s conviction that members of the ATR pool provide needed services to schools and that their work should be respected,” Mulgrew said in an emailed statement.

But critics argue that if principals had wanted to hire these teachers, they would have already done so. The result, they say, will put poor quality teachers into New York’s neediest classrooms.

“It is shockingly irresponsible for the city to force place hundreds of teachers of dubious quality into the classrooms of our most vulnerable students,” said StudentsFirstNY Executive Director Jenny Sedlis in a statement. “There are reasons why no principal has chosen to hire them and this policy is bad for kids, plain and simple.”