TORONTO -- Thousands of high school students returned to class today after weeks-long teacher strikes -- and they won't have to write final exams to complete their courses.

High schools in the Toronto-area regions of Durham and Peel and the Sudbury-area Rainbow District reopened today after the Ontario Labour Relations Board ruled the strikes illegal Tuesday evening.

As the more than 70,000 students headed back to class, Education Minister Liz Sandals wrote to the school board chairs, encouraging them to maximize the instructional and learning time remaining in the school year.

Sandals says final marks should be determined without exams and she is waiving the 110 hours of instruction requirement for secondary credits in the affected schools, as well Grade 9 standardized math tests will not be administered in those boards.

She says the school year will not be extended as it could interfere with students' plans for summer jobs.

The labour relations board ordered a two-week moratorium on the strikes, so the Liberal government is proceeding with back-to-work legislation, as it bans strikes in those boards for the rest of the school year.

Labour board chair Bernard Fishbein sided with the school boards in concluding the three local strikes were, at least in part, on the central issue of class sizes.

This is the first round of negotiations to formally separate local and central bargaining under legislation the Liberal government brought in last year.

Lawyers for the three school boards argued that although the law doesn't explicitly ban teachers from staging local strikes on provincial issues, that's what it was meant to do, and Fishbein agreed.