More than 70,000 students have been out of class for weeks and the school boards in Durham Region, Rainbow District, which includes the Sudbury area, and Peel Region say the strikes are unlawful.
The boards believe the Ontario Secondary School Teachers' Federation is staging local strikes on central issues, which 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 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.
The Labour Relations Board hearing is set to start at 10 a.m. and a decision is expected to come later that day or Friday.