Public schools in Ontario will lose approximately 10,000 teachers over the next five years due to an increase in class sizes, the province's fiscal watchdog said Thursday.

Financial Accountability Officer Peter Weltman said the move by the Progressive Conservative government will save the province about $2.8 billion over the same period, and a further $1 billion annually thereafter.

In its report, the FAO estimates there will be 9,060 fewer high school teachers and 994 fewer elementary teachers by the 2023-24 school year.

"The class sizes have been [increased] so those teacher jobs will no longer be required," Weltman said. "It's not a cut, it's a cost avoidance."

The Tory government has said the changes in class sizes were necessary to fight a $7.4-billion deficit and will result in 3,475 fewer teachers in the system over four years.

The FAO said the government arrived at that estimate by looking at the actual number of teaching jobs in 2018-2019 that won't exist four years from now. The financial watchdog, however, looked at the number of teachers that would have been required had class sizes not changed.

"We are always looking at changes against status quo," he said. "That's where we derived our 10,000 number from. That was based on a projection of student enrolment, increased population ... [and] how many teachers would that require in a 22-student class size versus a 28-student class size."

Earlier this year, the government ordered school boards to start increasing class sizes, moving to an average of 28 from 22 in high schools over four years. Class sizes for grades 4 to 8 will increase by one student per classroom over the same period, to 24 from 23.

Facing a backlash over the impact the changes would have on teacher jobs, the government created a $1.6-billion fund for school boards to ensure no teachers were laid off as a result of its plan.

The FAO estimates that the government has provided enough funding to ensure the class size targets will be met without teacher layoffs.

'No teacher will lose their job'

Education Minister Stephen Lecce said the report recognizes the government's plan to avoid layoffs is working.

"The officer confirmed what we have been saying all along: no teacher will lose their job as a result of our class size policy," he said in a statement.

The president of the Ontario Secondary School Teachers' Federation said the report shows the class-size changes will have a "massive" impact on classrooms.

"The government will claim there is some foundation for their numbers," Harvey Bischof said. "Really what they're doing is trying to create an impression that their policy changes will be less damaging to student's education than they really will be."

Bischof said the FAO numbers were entirely in line with the union's projections.

"I don't see how this government has a shred of credibility left when it comes to the education file," he said.

NDP education critic Marit Stiles said Premier Doug Ford's government is trying to balance the budget on the backs of students.

"This is really devastating news but it confirms what we've been afraid of ... a dramatic cut in the number of teachers in classrooms and the opportunities for our students," she said.

Green party Leader Mike Schreiner agreed that eliminating teaching jobs fails the province's children.

"Although the premier campaigned as if he would simply trim the fat, his cuts are digging into the bone," Schreiner said in a statement. "This is a direct threat to the quality of education in Ontario."