The pool of teachers collecting salaries and benefits without holding full-time positions shrunk by roughly 150 compared to this time last year, according to numbers released Thursday by the Department of Education.

The number stands at 1,304 this October — a decline driven by some teachers finding new jobs and others leaving the school system, city officials said. The size of the pool represents only a snapshot in time and fluctuates throughout the school year, but the Department of Education argues that, in the aggregate, the pool has been steadily decreasing.

“We must have a strong teacher in every classroom to provide an equitable and excellent education for all students,” said schools Chancellor Carmen Fariña. “We’re laser focused on ensuring this while reducing the ATR pool and costs for the city’s taxpayers.”

Mayor Bill de Blasio and the city’s teachers union have vowed to reduce the number of teachers in the Absent Teacher Reserve, both by offering buyouts and by helping qualified teachers find jobs. The teachers union contract included provisions to help ATR teachers interview at schools and a buyout that 115 teachers and other staffers took.

A UFT spokesperson said Thursday that the decline in ATR pool represents a “joint effort” between the union and the Department of Education.

City officials said they could not pinpoint exactly how many teachers received new job placements versus how many left the system entirely. When they city last released ATR data in February, officials reported they had successfully placed 500 teachers in full-time positions in both the fall of 2014 and 2015.

Not everyone is thrilled by that statistic. Reducing the ATR pool by putting more teachers in classrooms does a disservice to schools and students, said StudentsFirstNY, an advocacy group that backed Mayor Michael Bloomberg’s education policies. (The ATR grew under Bloomberg, costing the city about $105 million.)

“The de Blasio administration should not be cheered for shrinking the pool by placing ineffective teachers back in classrooms, which does a profound disservice to thousands of students,” said Jenny Sedlis, executive director of StudentsFirstNY.

Eric Nadelstern, a former deputy chancellor of New York City schools under Bloomberg who is now a professor at Teachers College, said another reason for the shrinking pool is far fewer schools are closing under de Blasio than under Bloomberg.

Teachers are typically placed in the ATR pool for budgetary reasons, Nadelstern said, or because they were performing poorly. Instead of keeping them within the school system, he said he would do the same as most other “sensible” industries and offer the teachers more buyouts.

“The thought of that in any other field of endeavor would be absurd,” Nadelstern said, “and yet we regularly treat teachers as if they are fungible and interchangeable — and they’re not.”