As 11 p.m. neared lawyers for the Elementary Teachers' Federation of Ontario were making their case for why the one-day protest should be allowed to go ahead.
Earlier the OLRB heard from lawyers for the province, arguing that the walkout is illegal.
The board is expected to rule on the question after hearing both sides.
ETFO represents 76,000 public elementary teachers and education professionals plans to have a day-long "political protest" on Friday that the government views as illegal.
In a bid to prevent that from occurring, the governing Liberals went to the OLRB seeking an injunction.
Early in the proceeding OLRB chair Bernard Fishbein denied the union’s request to defer the hearing.
Teachers furious over legislation
The planned protest is in retaliation for the government's move to impose two-year contracts on public elementary school teachers using Bill 115, the controversial legislation known as the Putting Students First Act.
Premier Dalton McGuinty has said that what the teachers' union has planned for Friday is, in the government's view, "illegal strike activity."
However, ETFO president Sam Hammond said Thursday that the premier's characterization of the pending walkout as illegal was simply wrong.
"I find it very interesting that the premier, who knows that he cannot determine whether this is a strike or a political protest, has made that decision already and … today is going to go to the Ontario Labour Relations Board to have them make that determination where it should be made," Hammond said during a telephone interview on CBC Radio's Metro Morning on Thursday morning.
But McGuinty again spoke with the media on Thursday, urging teachers to consider their actions before participating in the protest.
"I understand that we have some differences. I respect their right to give expression to those differences," he said. "I respect that they bring a different perspective to this issue, but we have a shared responsibility to resolve this difference outside of the school setting."
The premier said that teachers could voice their opposition before and after school, on holidays as well as on weekends — but not during school hours.
Toronto school closures
Meanwhile, the 474 elementary and junior-high schools run by the Toronto District School Board will be closed Friday even if the government is able to a prevent the walkout by teachers.
Even if teachers are forced back to work, there is no way to know exactly how many staff members will be able to come to work, according to a TDSB spokeswoman.
"We can't ensure that there's enough adult supervision at the school to make it a safe environment," said Shari Schwartz-Maltz.
The union says it does not view the protest as a strike and it is allowed under the Charter of Rights and Freedoms.
Teachers who do work Friday will be paid, and they will be asked to do professional development work and marking. Those who choose not to come will not be paid, said the TDSB.
The City of Toronto issued a release late Thursday, saying that city-run child-care centres "will be open as usual," even if they are located at TDSB sites. However, parents are urged to check with their local child-care centres.
The public 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.
The difference between those protests and the protest planned for Friday morning is that the December protests happened before the government imposed the new contract.
Meanwhile, the Ontario Secondary School Teachers' Federation said late Wednesday the 60,000 secondary school teachers it represents will be asked to hold "a day of political protest" next Wednesday, Jan. 16, if the government does not repeal Bill 115.
The Liberals had enjoyed a lengthy period of labour harmony with Ontario teachers, prior to the conflict that has erupted in recent months.
As McGuinty prepares to depart Queen’s Park, he admits that his last few weeks on the job have been tumultuous.
"Well, the teachers have kept my exit interesting for me, I'll say that much," he said Thursday.
McGuinty announced in October that he was stepping down. But he agreed to stay on as Liberal leader until the party chooses a successor, which will happen at a convention later this month.
Nina Bascia, a professor at the Ontario Institute for Studies in Education, said there is a question mark hanging over how the Liberal government will work with the teachers under a new leader – and if there will be any change at all.
"It's not clear what a new education minister would do, it's not clear how long Laurel Broten will be the education minister," Bascia told CBC News in an interview on Thursday.Suggest a correction