TORONTO -- Thousands of high school students returned to class Wednesday after weeks-long teacher strikes ended, though tensions between the teachers and the Liberal government have only increased.

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

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.

"The OLRB ruling, while it's gotten the kids back in the class and that's a good thing, it actually leaves it open to the teachers to resume their strike on June 10th and clearly that's not acceptable," said Education Minister Liz Sandals.

Minutes after she made those remarks the Ontario Secondary School Teachers' Federation issued a statement saying they would indeed resume the three local strikes on June 10. They will not be able to do so legally once the back-to-work legislation passes. A vote is expected Thursday.

"We emphatically maintain that these strikes have always been about local issues, and our members in Durham, Rainbow and Peel will be back on the picket lines on June 10," OSSTF president Paul Elliott said in a statement. He was not available for interviews.

Sandals said that "certainly explains" why the government is still going ahead with the back-to-work legislation.

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. The two-week moratorium was an opportunity for the union to "cleanse" the local strikes of central issues.

Sandals said the government is proceeding with the back-to-work legislation to ensure kids stay in school for the remainder of the year because the Education Relations Commission ruled that their years were in jeopardy. The teachers had been on strike for between three and five weeks.

NDP Leader Andrea Horwath said the back-to-work legislation is adding "fuel to the fire" and sends the message the government has no interest in bargaining. Central talks are stalled at the provincial level with both the high school and elementary teachers.

Progressive Conservative critic Garfield Dunlop said the legislation is akin to "putting your finger in a dam," suggesting the labour unrest will not be resolved -- and may even be worse -- come September.

As the more than 70,000 students headed back to class Wednesday, Sandals wrote to the school board chairs, encouraging them to maximize the instructional and learning time remaining in the school year, including cancelling their formal exam weeks. A requirement of 110 hours of instruction for secondary credits is also being waived in the affected schools.

The school year will not be extended as it could interfere with students' plans for summer jobs, Sandals said.