The Canadian Union of Public Employees, which represents 55,000 custodians, educational assistants and others, said it would escalate job action if the government then doesn't back down on Bill 115.
Their members make an average of $38,000 a year and many are laid off many times a year to match the school schedule, CUPE Ontario president Fred Hahn said Thursday.
"We are continuing to bargain, even as we speak," he said.
"But we are realistic. We know that we can't, even with our best efforts, get this job done by Dec. 31."
That's the deadline for public school teachers and other education workers to negotiate new collective agreements with school boards.
If they don't, the governing Liberals have the power to impose a deal on teachers that's similar to the one it struck with Catholic teachers — which will make any strike action illegal.
The deal with Catholic teachers froze the wages of most instructors and cut benefits, such as the banking of sick days to be cashed out at retirement.
Hahn won't say what kind of job action CUPE members might take if the government imposes a new agreement on them.
The Elementary Teachers' Federation of Ontario is also warning of a province-wide day of protest next year if the government uses Bill 115 to impose a contract on its members.
The union scheduled a news conference for Friday to announce details of its next steps.
Meanwhile, the union representing public high school teachers said its members are voting this week on whether to support a day of political protest against Bill 115.
But Premier Dalton McGuinty said he's optimistic that teachers can reach new agreements with their school boards in the next 11 days.
Ontario's self-described education premier, who will be stepping down at the end of January, says talking about labour unrest in schools isn't what he wanted to be doing at the end of his nine years in power.
"It's certainly not what I'd hoped to be doing," he said.
"But I remain optimistic. I still believe there is a tremendous amount of goodwill among our teachers and publicly funded education in the province of Ontario."
His government has done a lot for teachers, McGuinty added.
While the student population has dropped by six per cent, the teacher population has gone up 10 per cent, he said. His government hired 13,400 more teachers, which has reduced class sizes.
His government also increased teacher pay by 24 per cent, except for those who started work on his watch, in which case their pay increased 80 per cent as they moved up the grid.
Teachers are worth every penny, he said, but the province can't afford any more pay increases.
The teachers' unions say the dispute isn't over money, but McGuinty disagrees.
"No it's not. It's about pay," he said.
"I haven't heard of any public offering on the part of the federation leaders — any — that does not involve an additional cost. ... When somebody says it's not about the money, it's about the money."
The Progressive Conservatives say they agree: the dispute is about pay. But McGuinty's government is missing in action.
"They have legislation in place that they're ignoring," said education critic Lisa MacLeod.
"We now have seven mini-premiers in the leadership contenders in his party who are all saying different things. So there's no clear signal from this government where they stand."
Several high school students who came to the legislature to speak with NDP Leader Andrea Horwath said they're losing out on a lot in their final year of school.
"What they're taking away from us, our whole school environment, our whole school vibe is gone," said Braxton Wignall, 17. "Now at school, it's very dead."
Wignall, who attends SEED alternative school in Toronto, said he's applying to university, but his math skills are weak.
"I want the marks to go up, but if my teacher doesn't help, I don't get help anymore," he said. "So I'm staying there, I'm not advancing in the math curriculum, so it's tough."
The labour unrest in Ontario schools is affecting students the most, said Horwath.
There are reports that some teachers are facing disciplinary action for submitting incomplete reports.
McGuinty said teachers should fulfil all their professional obligations, including report cards.