TORONTO -- Ontario's biggest teachers' union is warning of major disruption in elementary schools across the province in December as teachers ramp up strike action to protest the governing Liberals' controversial anti-strike law.

Parents will get 72 hours notices of strikes planned next month, which will affect operations in each public elementary school throughout the province, the Elementary Teachers' Federation of Ontario said Wednesday.

A union spokeswoman confirmed that strike action could include teachers not being in class but would not provide further details.

"It is unfortunate that we have been placed in the position of having to strike by Education Minister Laurel Broten, but we will provide parents with ample notice to ensure the safety of students," ETFO president Sam Hammond said in a statement.

"By her actions, the minister has let everyone down by stripping teachers and education professionals of their democratic rights, and shutting down recent talks at the provincial level."

But Broten and Premier Dalton McGuinty are defending the unpopular legislation, which also freezes the wages of most teachers and cuts their benefits.

McGuinty insists the legislation is needed, even though it appears to be preventing some teachers from accepting new contracts.

High school teachers in York Region rejected a tentative agreement reached with their school board, reportedly because they felt they'd be "selling out" by accepting a deal they feel was imposed by the Liberals under the new legislation.

Many of the board's 2,800 teachers were reportedly concerned that it might also weaken a constitutional challenge of the legislation that's been launched by four unions.

It's possible some teachers are holding out for a new Liberal leader to repeal the law, as some of the candidates vying to replace McGuinty at the end of January have been critical of the legislation.

But McGuinty said he believes there's a consensus among Ontario residents that his cash-strapped government has to take "effective measures" to eliminate a $14.4-billion deficit, which includes wage freezes.

"We think it's the best approach, it's a responsible approach and I think it's one that's broadly supported by Ontarians," he said in Windsor.

While he wouldn't comment on whether the legislation is a roadblock to new contracts, McGuinty said he remains optimistic that other teachers will accept a deal, he said.

"We've had one ratified, a couple that have been rejected," McGuinty said. "We're working as hard as we can to lay the groundwork for some negotiated settlements."

High school teachers with the Upper Grand District School Board -- which covers Guelph, Dufferin County and Wellington County -- ratified an agreement reached with the school board and approved by the education minister.

But teachers in Niagara -- as well as York Region -- rejected theirs.

That means teachers will continue with strike actions that include skipping staff and department meetings, no parent-teacher conferences outside of school hours and not submitting student attendance records.

Meanwhile, elementary teachers and other education workers in four school boards have already started work-to-rule job action.

The ETFO -- which represents 76,000 teachers and education professionals -- says teachers in boards across the province will all be in a legal strike position by the Christmas break.

The Liberals have come under fire for not using their sweeping powers under the new law to stop strike action that's quickly spreading across the province.

Broten said she's disappointed with the new ETFO warning, but will continue to encourage teachers to reach local agreements with school boards before Dec. 31. After that, the province will be able to impose its own agreement on teachers.

"My message to parents is one that we will very much closely monitor the situation," she said.

"It is too soon to say which of the tools will be necessary. But we do have precise tools to prevent strike action that might jeopardize, as an example, student safety -- like teachers refusing supervision."

More contracts would have been signed by now if the Liberals had never introduced the legislation, said NDP education critic Peter Tabuns.

"If the government had been willing to bargain hard, roll up their sleeves, the teachers would have been in the same frame of mind," he said.

"Everyone knew it was a difficult time. They would have worked it through and come to something that people could have lived with."

But McGuinty said he's optimistic that other teachers will accept a deal.

"We've had one ratified, a couple that have been rejected," he said. "We're working as hard as we can to lay the groundwork for some negotiated settlements."