TORONTO -- More than 70,000 Ontario high school students were kept from school Monday as teachers in three boards hit the picket lines in a labour environment growing more tenuous by the day.

High school teachers in the Toronto-area Peel Region, where there are 42,000 students, went on strike Monday, joining Rainbow District, encompassing Sudbury and its surrounding regions, and Durham Region, east of Toronto, where 24,000 students have been out of class since April 20.

The Peel talks to avert a strike went into the early-morning hours Monday, but ultimately failed to reach a deal. At the same time, central talks between the Ontario Secondary School Teachers' Federation broke down. Elementary teachers are in a legal position to launch a province-wide strike Sunday.

This is the first round of negotiations since the province brought in a new bargaining system with both local and provincial talks, although the head of the Peel school board believes the strike is not about local issues at all, but a "provincial takeover disguised as a local action."

"I am absolutely convinced that this is a central issue," she said. "I was at the bargaining table for four days and four nights. I'm well acquainted with the local issues and I'm absolutely convinced we cannot reach a local deal without the central table settling first."

Education Minister Liz Sandals has said she is "perplexed" by what local issues could be prompting the strikes. She described them Monday as "technically local strikes" and said she was optimistic a central deal could be reached.

"Progress is gradually inching forward," she said.

But the OSSTF said no further bargaining dates have been scheduled with the province after talks reached a late-night impasse. A sticking point is class sizes, said union president Paul Elliott, who suggested the province wants to remove class-size caps.

Monetary issues are being dealt with centrally at a time when the province, facing a $10.9-billion debt, has said any compensation increases must be "net zero." But the negotiation "frustrations" are not about money, Elliott said.

"We recognize the government's fiscal issues," he said. "We're talking about working conditions. We're talking about learning conditions."

In Peel, he said a major outstanding issue is parameters around when and how teacher performance appraisals are done.

Progressive Conservative education critic Garfield Dunlop said the new bargaining system is a "train wreck."

"I think what we've seen very clear over the last three or four weeks now is the two-tiered system is not working," he said. "Now you've got 72,000 students that are out of the classroom right now...(elementary) students could leave next Monday. I think it's completely irresponsible to think this system's actually working."