Budget cuts caused principals to cut thousands of positions this year, but the total number of teachers without permanent jobs rose only slightly, the Department of Education revealed today.

The Bloomberg administration also announced plans to lay off nearly 800 school employees who do not belong to the teachers union, which negotiated a deal in June to avert layoffs. Most of those employees — 737 of 777 — belong to DC-37, which represents school aides and other auxiliary school personnel. The layoffs are set to start in October.

When the city announced in July that schools would have to cut an average of 2.43 percent from their budgets, many principals complained that they had little fat to trim. They said they would have to turn to eliminating necessary positions and sending junior teachers to the Absent Teacher Reserve, the pool of teachers whose positions were cut or lost as a result of school closures or enrollment changes.

In the end, they sent 2,186 teachers to the ATR pool this summer. More than a thousand of those teachers have already left the pool, either by finding new positions or leaving the system. A DOE spokeswoman said many of the teachers were rehired by their original schools after funding became available to keep them there.

That leaves 1,940 teachers in the ATR pool with just weeks before the start of the school year.  Last year, the pool contained 1,779 teachers just before classes began.

Though small, the growth in the size of the ATR pool still places added financial stress on the department. That’s because teachers in the pool draw their regular salary even as they work in temporary positions, this year changing schools weekly as the result of the city’s budget deal with the teachers union. Ex-Chancellor Joel Klein’s last message to principals before he left the DOE took aim at the cost of maintaining the ATR pool: He asked for permission to lay off the reserve teachers, saying that the city was spending as much as $100 million a year to support teachers who “don’t care to, or can’t, find a job.”

Teachers union officials speculated that the excess numbers rose only slightly because more teachers exited the system completely this year. Nearly 2,500 teachers have retired this summer, 23 percent more than last year, the union reported.

The city had speculated that 2,600 jobs would be lost to attrition this year. Attrition includes retirements, resignations, and terminations.

The layoffs announcement surprised union officials with the most at stake. Lilian Roberts, DC-37’s executive director, said she had not received “official notification of layoffs” from the DOE and was “deeply concerned.” She added that she remained “hopeful that both sides will look for resources to avoid the need for any layoff.”

Marc LaVorgna, a City Hall spokesman, said in a statement that the Bloomberg administration that the layoffs were necessary because the unions had not signed on to cost-cutting concessions. “Unfortunately in this case, the unions involved would not agree to any real savings that could have saved these jobs,” he said.