TORONTO -- Three Ontario school boards where high school teachers have been on strike for weeks, affecting more than 70,000 students, are asking for the job actions to be declared unlawful.

In Durham Region, where secondary teachers are into their fourth week on strike, the board said it "strongly believes" it is illegal.

"It has become increasingly apparent through the language and action of the union leaders that these three strikes are related to the larger central issues and that (Ontario Secondary School Teachers' Federation) has put (Durham) teachers on a local strike based on central issues," the board wrote in a statement.

That is not allowed under a new bargaining system the Liberal government introduced last year. It separates local and central negotiating, with issues such as money and class sizes being discussed at the provincial table.

The Durham District School Board, east of Toronto, the Rainbow District School Board, in the Sudbury area, and the Peel District School Board, west of Toronto, are filing a joint application to the Ontario Labour Relations Board, asking it to declare the strikes unlawful.

"We've said, from the beginning, what we know is true: that provincial OSSTF is setting the agenda for local bargaining and that this local strike is part of their overall provincial strategy," Peel District School Board chair Janet McDougald said in a statement.

"We've seen secondary teachers in each of the three boards protest issues being negotiated at the central table, particularly the central matter of class size. Our teachers need to know, and our parents and students need to know, that there is nothing we can do at our local table to impact class size decisions -- nothing."

The head of the Rainbow District School Board said in a statement that there are misconceptions that the board is trying to increase class sizes, limit prep time and freeze salaries and benefits at the local table.

"Those issues are not part of the local negotiation process. They are central issues," chair Doreen Dewar wrote.

"We have maintained from the outset that our students and their families are caught in a dispute that is really between OSSTF and the province."

Central talks between OSSTF and the province are stalled.

Ontario's public elementary school teachers began a work-to-rule strike Monday. During their job action schools will remain open and extracurricular activities and field trips continue for now, but teachers won't administer standardized tests, add comments to report cards or participate in any professional development related to Ministry of Education initiatives.