TORONTO -- More than 70,000 high school students kept from class for weeks by teacher strikes are set to return to school Wednesday after the Ontario Labour Relations Board ruled the job actions illegal.

The ruling came Tuesday evening, as the Ontario legislature began debate that was set to last until midnight on back-to-work legislation for the striking teachers.

Education Minister Liz Sandals said government lawyers were combing through the labour board ruling to determine its impact on the back-to-work legislation, but in the meantime the Liberal government would continue to try to get it passed quickly. A vote is expected on Thursday.

But school boards in the Toronto-area regions of Durham and Peel and the Sudbury-area Rainbow District wasted little time in publicly announcing they would reopen schools Wednesday.

The labour relations board ordered an immediate two-week moratorium on the strikes to give the Ontario Secondary School Teachers' Federation an opportunity to "cleanse" the local strikes of central issues. It does not permanently prohibit local strikes, whereas the back-to-work legislation prohibits strikes in those boards for the rest of the school year.

Labour board chair Bernard Fishbein sided with the school boards in concluding the three local strikes were, at least in part, on the central issue of class sizes. There was no "smoking gun," he wrote, but from the evidence he heard class size was "certainly" an issue on the picket line.

It would not have been enough for a "scintilla or a whiff of evidence" that the strike was about central issues, nor would a single picket sign about class sizes "fatally contaminate an otherwise proper, lawful, local strike," Fishbein wrote, saying this kind of decision will have to be on a case-by-case basis.

This is the first round of negotiations to formally separate local and central bargaining under legislation the Liberal government brought in last year.

Lawyers for the three school boards argued that although the law doesn't explicitly ban teachers from staging local strikes on provincial issues, that's what it was meant to do, and Fishbein agreed.

"If the (act) and its two-tier bargaining is to have any meaning, the parties must respect the central/local split (particularly when they themselves have agreed to it), and take some responsibility to ensure that the (act) is complied with," he wrote.

"I do not believe it too onerous a burden on trade unions that choose to engage in local strikes while central bargaining is ongoing, to say they should be vigilant in ensuring their local strikes cannot be construed as being 'in respect of central bargaining."'

Fishbein took the opportunity to write in his decision that he heard little to no evidence of communication between the OSSTF and its members in the striking district about what the local issues are.

Premier Kathleen Wynne said Tuesday that once this round of negotiations with teachers is over, she's open to making changes to the process.

"Right now we're in a difficult negotiation because we're operating in a net zero environment where there isn't new money for compensation and at the same time we are using this new process," she said Tuesday in Hamilton.

"If there are changes we need to make to the process once we're through this, then we can look at that."

The NDP called on Wynne to fire Sandals.

Progressive Conservative Leader Patrick Brown called the new negotiating system a "two-tier disaster."

"It's not working," he said, suggesting it's difficult to have both central and local negotiations ongoing at the same time. "I think it's flawed and frankly the government should go back to the drawing board."

Sandals said the concept of two-tier bargaining is fairly common.

"In most provinces there's one process to sort out the money with the province and there's another process to sort out local collective agreement language, so that's not an unusual situation," she said.