TORONTO -- Ontario's public education system could be thrown into turmoil in September as the province's major teachers' unions have raised the possibility of strikes, setting the stage for a tense summer of bargaining.

More than 70,000 high school students returned to class Wednesday after the Ontario Labour Relations Board ruled that the teachers' strikes in the Toronto-area regions of Durham and Peel and the Sudbury-area Rainbow District were illegal -- a decision that would only ban the strikes for two weeks.

In an attempt to avoid further strikes in those boards and ensure the students would be able to complete their school year, the Liberal government introduced back-to-work legislation, which passed Thursday.

However, the new legislation does nothing to quell the potential unrest looming for the fall semester.

Education Minister Liz Sandals acknowledged that the Ontario Secondary School Teachers' Federation is "upset," but the fact the union is not at the bargaining table at the moment, she said, doesn't necessarily mean a central deal can't be reached over the summer.

"Quite frankly I'm fine with having a cooling-off period before we go back to the table with OSSTF," Sandals said. "It's usually better to talk to people and negotiate when they're calm rather than when they're upset."

The province's talks with the OSSTF broke down last weekend and the union has applied for conciliation, which could set the stage for a provincial strike. Union president Paul Elliott has said he is looking at the possibility of a strike in the fall.

Elementary teachers have been on an administrative strike since May 11 and their union promised an update Friday on their job action. Negotiations with the province are at a standstill.

The province does have bargaining dates scheduled with the Ontario English Catholic Teachers' Association with a mediator. While the head of that union said he is "hopeful" a strike can be avoided, a work-to-rule campaign may be in the cards.

"In the event that we don't make progress at the table and the school boards and the province continue to push for the concessions to our collective agreements...I don't see how September would be possible without some form of action," said OECTA president James Ryan.

The Catholic teachers' union lists many of the same sticking points as the other major unions: class sizes, control over teacher preparation time and hiring practices.

Progressive Conservative critic Garfield Dunlop said the Liberal government has accomplished little at the bargaining tables in several months.

"We've got a far bigger problem coming up...We've got all the school boards prepared to do either disruptive work to rule or all-out strikes come Sept. 8," he said.

"I think that's entirely possible and I think they're underestimating. They think they can negotiate this thing in 100 days...I don't think it's going to happen."

NDP Leader Andrea Horwath said she hopes the government has learned its lesson and negotiates, rather than legislates.