TORONTO -- Thousands of Ontario teachers rallied outside the provincial legislature Thursday as the province's Labour Relations Board met to consider whether strikes by high school teachers in three boards are illegal.

More than 70,000 students have been out of class for weeks as high school teachers in Durham Region, Rainbow District, which includes the Sudbury area, and Peel Region are on strike. The chanting crowd, holding up signs and union flags, included a mix of striking high school teachers and their elementary school colleagues, who began a work-to-rule strike this week.

"This premier promised us -- promised us -- a year ago that she would do nothing to strip our collective agreements and now she's turning around and trying to do it," said Mary Karchemny, an elementary school occasional teacher in Waterloo Region.

She said many feel "very disheartened" at the tone of discussions with the province, adding it's important for teachers across various unions to stand together.

Morale could be further affected if the Labour Relations Board mandates teachers back to work, said James Clyke, president of the Rainbow District local of the Ontario Secondary School Teachers' Federation.

"If they are successful in overturning the strike, I'm not quite sure what's going to happen," he said. "It's not a very good relationship."

Education Minister Liz Sandals said the protest "really doesn't change anything" as far as the province is concerned.

"Our resolve is to carry on with negotiations and conclude negotiated collective agreements," she said.

But the negotiations between the province and the central OSSTF are not carrying on -- talks have been at an impasse for weeks.

This is the first round of negotiations under a new bargaining system the Liberal government introduced last year, separating the process into local and central talks. The local high school teachers' unions that are on strike are negotiating with their respective school boards, not the provincial government.

But the school boards believe the OSSTF is staging local strikes on central issues, such as money and class sizes, which is not allowed under the new system. They're asking the Ontario Labour Relations Board to declare the strike illegal.

The labour hearing got off to a slow start Thursday, bogged down by procedural issues for hours, and the board planned to continue the hearing on Friday.

The local unions deny the school boards' assertion that the local strikes are just part of an overall provincial strategy to put pressure on the government.

Elementary teachers are not administering standardized tests, adding comments to report cards or participating in any professional development related to Ministry of Education initiatives -- including training on the province's new sex ed curriculum.

Schools remain open and extracurricular activities and field trips continue for now, but the president of the Elementary Teachers' Federation of Ontario said teachers may escalate the strike later.