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 asks 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. It is unclear when students with Rainbow District School Board will return to class.
The strikes have put more than 70,000 high schoolers in the Durham, Peel and Sudbury regions out of class for six weeks.
Read the labour board's decision below.