TORONTO -- A new Ontario law on school board bargaining came under fire Wednesday at a provincial labour board hearing that is expected to determine the legality of ongoing strikes by high school teachers.

The School Boards' Collective Bargaining Act -- passed last year -- was intended to bring clarity to labour negotiations, separating the bargaining process into local and central talks.

However, the boards and the teachers' union have been fighting for weeks on whether the strikes in the regions of Peel, Durham and the Sudbury-area Rainbow District violate the act.

Lawyers for the government and three boards where the strikes are taking place told the Ontario Labour Relations Board that although the law doesn't explicitly ban teachers from staging local strikes on provincial issues, that's what it was meant to do.

The school boards claim the teachers and their union have been largely protesting about class sizes, which is a central issue, and in the boards' opinion, the act prevents local strikes on central issues.

The strikes are "orchestrated" by the Ontario Secondary School Teachers' Federation, said school boards' lawyer Michael Hines, adding that they would be easier to fight if the act simply said local strikes on central issues are not allowed.

"Sure would," replied board chair Bernard Fishbein. "(We) probably wouldn't be here."

Heather Alden, the OSSTF lawyer, said the school boards are "in essence asking the board to read into the act a requirement that is not there, a requirement to scrutinize the conscience, hearts and minds of striking workers -- a task which is as invasive as it is impossible."

The sometimes contentious hearing focused Wednesday on the wording of one section of the act, dealing with notice teachers or support staff must give before a strike.

Employees must give at least five days' notice before a strike "in respect of central bargaining," the act says, with the following section setting out the same notice requirement for a strike on local bargaining.

Because these strikes are on central issues, the school boards argue, they are illegal because OSSTF didn't give proper five days' notice for a central strike, Hines suggested.

More than 70,000 students remain out of class while the labour board hearing, which began a week ago, was expected to continue into Thursday.

Fishbein said he will not rule immediately.

Education Minister Liz Sandals has also asked a separate arm of the labour board to consider whether the students' school year is in jeopardy.

Sandals has not said if the province is considering back-to-work legislation.