Some of Ontario's largest school boards have cancelled classes at public elementary schools for Friday, in advance of a "political protest" involving tens of thousands of teachers who are furious with the provincial government for imposing contracts.

On Wednesday evening, outgoing Premier Dalton McGuinty held a news conference warning the members of the Elementary Teachers’ Federation of Ontario (ETFO) that if they are not in school on Friday, they will be participating in an “illegal” strike action.

"A strike on Friday would be an illegal strike," McGuinty said.

"I know teachers are law-abiding. I know they do not want to break the law. And I am urging them not to."

The premier said it is the government’s "full expectation that teachers will be in school on Friday and every day in keeping with their employment obligations.”

To prevent a protest from proceeding, McGuinty said the government will go to the Ontario Labour Relations Board with an application in an attempt to "prevent the threatened illegal strike activity."

The group of Liberals seeking to succeed McGuinty backed up his decision.

"The action that they're taking is not within the bounds of the law, you know, and I think the premier is doing what he needs to do," said Kathleen Wynne after a candidate debate organized by the party Wednesday night.

"It's illegal action and that's not what the teachers should be doing. We have other ways by which they could do their protest," said Charles Sousa.

Reacting to the premier’s comments, NDP education critic Peter Tabuns said the conflict involving the teachers is proof that Bill 115 — the controversial Putting Students First Act — is not working, despite government claims to the contrary.

"His unconstitutional and shortsighted legislation has brought turmoil to our schools and he still acts as though it's working well. It isn't," Tabuns said in a statement sent to the media after the premier spoke on Wednesday evening.

Secondary school teachers plans 'day of action' too

Late Wednesday evening, the union representing 60,000 secondary school teachers in the province said it was also going to take its anger to the streets.

The OSSTF said in a news release it intends to ask its membership to hold "a day of political protest" next Wednesday, Jan. 16, "if the government has not repealed Bill 115, rescinded the Order in Council that imposed the OECTA MoU and restored free collective bargaining by that date."

The Toronto District School Board announced that its 474 elementary and junior-high schools would be closed Friday, and in Eastern Ontario, the Ottawa-Carleton District School Board followed suit.

West of Toronto, the Peel District School Board also announced that its elementary schools would be closed to students. The same decision was made by the York Region District School Board.

Elementary schools within the Hamilton-Wentworth District School Board will also be closed Friday.

In southwestern Ontario, the Greater Essex County District School Board said its elementary schools would also be closed to students.

Elementary classes are expected to be cancelled for public boards across Ontario.

ETFO, which represents 76,000 teachers and education professionals, issued a news release Wednesday saying that its members voted overwhelmingly last month in favour of a one-day protest if the government used contentious legislation to impose contracts.

The union says that when Education Minister Laurel Broten announced last week that the government would impose two-year contracts, the die was cast.

ETFO president Sam Hammond is seen speaking with members of the media Wednesday, the same day the union announced plans for a day-long political protest on Friday. (CBC)

"The minister made a deliberate and provocative choice to wipe out the democratic rights of tens of thousands of educators rather than work towards a respectful solution," ETFO president Sam Hammond said in a statement.

"She could have taken our olive branch and waited for a new leader to try and find solutions, but she chose not to."

At a news conference, Hammond said it was the view of ETFO that the protest planned on Friday was not a strike and is allowed under the Charter of Rights and Freedoms.

Relationship soured

The elementary teachers had previously launched a wave of rotating, day-long strikes in December, in protest against Bill 115, the legislation that gave the government the power to impose the contracts.

On Wednesday, Broten told reporters she would encourage ETFO members "to be fully apprised" of the collective agreement that has been imposed "and to understand that any strike action would be illegal activity."

But imposing any penalties on striking teachers would be up to the Ontario Labour Relations Board, Broten said.

Engaging in illegal strike activity can carry a penalty of up to $2,000 per person and $20,000 for a trade union, according to the Ontario Labour Relations Board.Fines would only be levied if the board determined it was an unlawful strike, gave consent to prosecute and a court agreed with the board's finding.

Until recent months, the governing Liberals had a strong relationship with public-sector teachers, which McGuinty made reference to in his remarks on Wednesday evening.

But teachers were protesting in Toronto on Wednesday evening outside the location where seven Liberal candidates were debating in an effort to succeed McGuinty at a leadership convention later this month.

The ETFO president said Wednesday that he wants the opportunity to converse with the new leader, to talk about the union’s concerns with the government.

With files from The Canadian Press