Elementary teachers in York Region, which includes 120,000 students in nine municipalities north of Toronto, started the first stage of what the union described as an "escalating withdrawal" of administrative services.
Teachers won't be arriving until 30 minutes before class starts Monday and Tuesday and will leave no later than 30 minutes after students are dismissed for the day, according to the local unit of the Elementary Teachers' Federation of Ontario.
"In most schools, it won't affect student safety," said Geoff Williams, a spokesman for the Ontario Public School Boards' Association.
"What it will do is perhaps limit the access of parents to teachers."
More teachers will be in a legal strike position over the next few weeks. All of ETFO's 76,000 members will be in a legal strike position within the next two to three weeks, the union said.
About 35,000 public high school education workers and teachers in 28 schools boards across the province are also taking strike action, including those at Toronto District, Ontario's largest school board.
However, high school teachers in five school boards have pulled back after reaching tentative agreements with their employers.
The deals were reached in York Region, Upper Grand, Hamilton-Wentworth, Thames Valley and Niagara District school boards, but they must still be approved by the government, said the Ontario Secondary School Teachers' Federation.
But office, clerical, technical and professional student services personnel in Hamilton-Wentworth — who still don't have a tentative deal — took strike action Monday.
Many high school teachers started strike action Nov. 12 after the OSSTF failed to reach a last-minute deal with the governing Liberals.
Education Minister Laurel Broten said she's pleased that the five deals were reached.
"I look forward to receiving the agreements in the days ahead to confirm that they meet the substantively identical test laid out in the Putting Students First Act," she said in a statement, referring to the anti-strike legislation that's sparked anger among teachers.
Four unions are taking the government to court, arguing the law is unconstitutional and violates collective bargaining rights.
Bill 115, which passed with support from the Progressive Conservatives, gave the government the power to stop strikes and lockouts, and impose its own agreement if it doesn't like what the unions and school boards negotiate together.
The government says any tentative deal must be similar to the one it struck with English Catholic teachers, which froze the wages of most teachers and cut benefits, such as the banking of sick days that can be cashed out at retirement.
While it's encouraging that deals have been reached, they may not signal the end of the labour strife in schools, said NDP education critic Peter Tabuns.
"I think (the deals) would have happened sooner if the provincial Liberals hadn't jumped in with Bill 115 and poisoned the well early on," he said.
It's too early yet to know if this may be a turning point in the province's battle with teachers, said Williams.
"The fact that five boards have reached deals, I think, is significant. I think it's a sign for optimism," he said.
"But I know that there are several more steps that have to be gone through before we could feel confident that it really was a good sign."
Progressive Conservative education critic Lisa MacLeod said the governing Liberals aren't doing enough to stop the strike action among elementary and high-school teachers.
They promised parents that the school year would be free from labour strife if the bill passed, but aren't using their new powers to stop the strike action, she said.
"Can you explain to me why the Liberal government recalled the legislature early to pass a law it has no intention of following?" MacLeod wrote Sunday in an open letter to Broten.