The head of Ontario’s high-school teachers’ union has vowed to “cultivate resistance” to changes being proposed by Premier Doug Ford’s government to the provincial education system.

In a fiery speech on Thursday, Harvey Bischof, president of the Ontario Secondary School Teachers’ Federation (OSSTF), said the Progressive Conservative government has “declared war” on the education system by making changes that would increase class-size averages in high schools and result in the loss of thousands of teaching positions and elective courses.

Open this photo in gallery Toronto high-school students stage a walkout on April 4, 2019. The head of the Ontario high-school teachers' union says it is the job of educators to 'cultivate resistance' to the Ford government's education policies. Chris Young

His address to more than 400 members attending the union’s annual conference in Toronto comes ahead of negotiations with the government. Teacher contracts expire on Aug. 31.

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“The government at Queen’s Park has declared war on one of the best education systems in the world. … That system is in peril, and we, as educators, are under attack as well,” Mr. Bischof told the audience.

“Our job is to cultivate resistance,” he added.

Mr. Bischof said teachers and support staff will be in the classroom on the first day of school. But he said that he is not optimistic about this round of bargaining. He declined to speculate on job action in the coming academic year.

“Students will already be suffering losses come September. We’ve already seen courses cancelled, we’ve seen support staff laid off. Am I optimistic? I think that would be a stretch. I’m very concerned that we’ve got a government that doesn’t recognize the investment value of public education in Ontario,” Mr. Bischof told reporters after his speech.

Initial discussions with the government and the OSSTF hit a snag, and both sides are heading to the Ontario Labour Relations board next week to determine which issues should be bargained centrally and what should be negotiated between local unions and individual school boards.

Education Minister Stephen Lecce said he was cautiously optimistic that a deal with the province’s teachers and support staff could be reached to keep the coming school year free of labour disruption.

“As certain union leaders are preparing for battle … I and the ministry and this government and the Premier are preparing for the first day of class,” Mr. Lecce told reporters before heading into a cabinet meeting on Thursday. “ … I want to make sure that we continue to focus and negotiate at the table in good faith.”

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Asked if the government’s proposed legislation that could cap wage hikes and its move to increase class sizes had affected the talks, Mr. Lecce said he believes a deal can still be reached: “I think we can still land a deal, to be quite frank. I want to be cautiously optimistic.”

There has been a lot of uncertainty in the education sector after Mr. Ford’s government announced a plan to increase average class sizes by one student in Grades 4 to 8, and to 28 from 22 in high schools. The changes would eliminate an estimated 3,475 teaching positions across the province over the next four years.

The government has stressed that there would be no layoffs and that teaching positions would be lost through attrition, meaning that educators who retire or voluntarily leave their job would not be replaced.

Still, several boards, including the Toronto District School Board and the York Region District School Board, have announced that a number of courses would be cut or scaled back as a result of the changes to class sizes, including courses in history, science and geography. Others have also said that the changes would result in fewer specialized course options for students and could ultimately affect graduation rates.

The government has said that it would be providing school boards with $1.6-billion in transitional funding, and that that should be used to protect specialized programs.