With strikes looming, the first as early as Monday, Ontario's public high school teachers are balking at a proposal to fit more students into their classes and more non-classroom duties into their workdays.

The province wants to eliminate caps on class sizes – established by the last Liberal government – and that's partly why the Ontario Secondary School Teachers' Federation walked away from the bargaining table this week, The Globe has learned.

Central negotiations with the province have stayed secret, even as Durham District School Board prepares to close its high schools on Monday, and as six other boards remain on notice that they may face their own strikes this spring.

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However, the union and the association of school boards have both confirmed two major sticking points that contributed to the breakdown of talks on Wednesday. The talks hadn't resumed as of Friday evening.

Former premier Dalton McGuinty made it his mission to impose smaller class sizes. Currently, boards must maintain an average of 22 students per high-school class.

But the current Liberal government is grappling with a $10.9-billion deficit. Under legislation passed last year, it is negotiating with the teachers' union at a central bargaining table on big-ticket items with major financial implications, while each board negotiates over smaller items. A stalled central negotiation increases the risk of local strikes.

Michael Barrett, head of the Ontario Public School Boards' Association, which represents board managers, said the province's offer is a "net zero" contract in which salary increases must be offset by other sacrifices.

He said flexible class sizes means savings; rather than raising the caps, the board association and province want to eliminate them.

"The concept of hard caps is a very difficult thing for school boards to work with," Mr. Barrett said.

Imagine a class with a cap of 23, and one more student arrives, he said. "Does that mean that you have two classes of 12?" he said. "Or do you have a guideline that says that you can put an extra child in a class of 23? Because you're hiring an extra teacher."

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The province declined to speak about the talks.

Paul Elliott, president of the Ontario Secondary School Teachers' Federation (OSSTF), said most university-stream classes already have more than 30 students.

"We have worked for years to really minimize the size of classes," Mr. Elliott said. "The larger a class is, and the more a student needs support, the less opportunity there is to provide that support in a larger class."

Another major sticking point is asking teachers to increase the time spent supervising students at lunch, in the hallways and in classes when other teachers are absent, he said. They're being asked asked to do 60 of these half-periods per year, which more than doubles what many teachers now do.

"[Teachers] are not responding well," Mr. Elliott said. "They really see that as their self-directed time."

He said teachers work an average of 56 hours a week in and outside the classroom – they are paid by the day – and spare periods go toward marking, preparing classes, talking to parents and meeting privately with students, he said.

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However, Mr. Barrett said the measure would allow boards to hire fewer substitute teachers and lunch monitors; besides, teachers used to do those tasks more often, he said.

"It's just because the amount of time that has been allocated to that has been shrinking over the last number of contracts, and it gets more and more difficult to be able to ensure that students are supervised," he said. "They're not working a longer day."

As for salaries, Mr. Elliott said that question had barely come up. But Mr. Barrett said the teachers' starting offer included a pay hike that "doesn't match the government's concept of a net zero increase." He believes salaries are more important to teachers than their union lets on, based on what he sees "in Twitter-land."

Outside of Durham, the Rainbow District School Board in Sudbury is the only one with a set strike date: April 27, according to the OSSTF. The other five boards that have been served notice that teachers may strike are Halton, Peel, Waterloo, Ottawa-Carleton and Lakehead in Thunder Bay. The union must give at least a week's notice to each board if it intends to strike.

Mr. Barrett, who is also chair of the Durham board, said this week that high schools would be closed entirely in event of a strike, but that the board was working to plan some support for families with special-needs students.