The government of Ontario has passed a controversial law that could impose a fine of C$4,000 ($2,900; £2,600) a day for workers who go on strike.
To pass the bill, the government invoked a special clause that allows it to bypass constitutional challenges.
The Canadian Union of Public Employees (CUPE) responded with defiance, saying workers would strike, regardless.
The legislation would also allow for a C$500,000 a day fine against the union itself.
CUPE represents some 55,000 education workers, including educational assistants, custodians and administrative staff.
The union originally asked for an 11.7% annual pay rise for its employees across the board, but sources told local media the ask had been dropped to roughly 6% during last-minute negotiations.
The government countered with an offer of 2.5% for workers making less than C$43,000, and 1.5% for those earning more. The average inflation rate is 7%.
Union leaders have called for “province-wide political protest” starting on Friday after the government introduced Bill 28 – which enforces the daily fines for a four-year period.
They have promised to pay the fines of those on strike.
“Our members will not have their rights legislated away,” said Fred Hahn, President of CUPE Ontario. “Now’s the time to stand up for ourselves and public education and that’s just what we’re going to do.”
The Ontario Public Services Union – another union representing some 8,000 workers – said its members would join the picket line in solidarity with CUPE counterparts.
“Bill 28 isn’t just an attack on education workers’ collective bargaining rights, it is an attack on all workers’ rights,” its president JP Hornick said.
The Toronto District School Board said its schools would be closed for in-person learning for all students for the duration of the strike action.
Ontario Premier Doug Ford has said the measure is necessary because “shutting down classrooms would have an unacceptable impact on students”.
The country’s Prime Minister, Justin Trudeau, phoned Mr Ford on Wednesday to describe his plan as “wrong and inappropriate”.
Bill 28 has been introduced via a rare legal manoeuvre called the notwithstanding clause, even though Mr Ford’s government acknowledges it breaches the Canadian Charter of Rights and Freedom.
The clause empowers provincial governments to override portions of the charter for a five-year term as long as they are “demonstrably justified in a free and democratic society”. Mr Ford also invoked it to pass legislation limiting third-party political advertisements ahead of the 2022 provincial election, which he won.
In 2018, he said he would use it to drastically cut the size of Toronto’s city council, but a court ruling giving him wide authority on matters of municipal policy meant he did not need to use it.
It was used in 2019, when Quebec passed a law barring public servants from wearing religious symbols like crosses, kippahs, turbans or hijabs.