The strikes by Ontario high school teachers in the school board districts of Durham, Rainbow and Peel are illegal, the Ontario Labour Relations Board ruled on Tuesday.

The labour board determined that the local strikes at the three school boards are in contravention of the School Boards Collective Bargaining Act.

The labour board also asked the Ontario Secondary School Teachers' Federation, the union representing the teachers in the three boards, to "cease and desist their unlawful strike" starting from Tuesday.

Following the ruling, Peel and Durham school boards said students and teachers will return to schools on Wednesday. Students at Sudbury's Rainbow District School Board will also return to class.

The strikes have kept more than 70,000 high school students out of class for six weeks.

Precedent-setting ruling

Janet McDougald, chair of the Peel District School Board, called the labour board's decision a "precedent-setting ruling."

"We're pleased that the OLRB's ruling supports what we've believed all along — that provincial OSSTF began an unlawful strike in Peel to put pressure on provincial negotiations," she said in a statement.

Because the labour board's decision was announced late in the day, reopening school would be a complex process, McDougald added.

"We know that with this little notice it won't be a perfect return to school tomorrow, but our incredible staff will make sure it is the best possible one for our students," she said.

The Ontario Ministry of Education said it is currently reviewing the ruling and will be determining next steps.

"Part of the determination will be hearing how OSSTF responds to the ruling," Education Minister Liz Sandals said in a statement.

Read the labour board's decision below