Ontario's largest teachers' union is asking a court to stop the provincial government from forcing educators to use a 20-year-old sex-education curriculum and to cancel a "snitch line" for parents to anonymously report concerns about what is being taught in classrooms.

The Elementary Teachers' Federation of Ontario (ETFO) filed an application for judicial review in Superior Court on Tuesday – the first day of school for almost all students – arguing that Doug Ford's government has acted unreasonably and infringed on the rights of educators under the Canadian Charter of Rights and Freedoms to express themselves freely.

"The Ford government has thrown the system into uncertainty," Sam Hammond, president of ETFO, said at a media conference on Tuesday. "The actions of the government to rescind the 2015 curriculum undermines the very safety and well-being of Ontario students and families.”

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Sam Hammond, president of the Elementary Teachers Federation of Ontario. (File Photo).

Kevin Van Paassen/The Globe and Mail

Last month, the Progressive Conservative government said that elementary school students would learn from a “revised, interim curriculum,” based on a 1998 document on sexual health, while it consults on a new one. The curriculum it is replacing, implemented in 2015 by the previous Liberal government, was more detailed and included issues such as consent, same-sex relationships, gender identity and the dangers of sexting.

By scrapping the updated sex-ed curriculum in the elementary-school years, Mr. Ford fulfilled a campaign promise (the government said it was not making any changes to the modernized high-school curriculum). Some parents and groups, especially social conservatives, had labelled it age-inappropriate for young children, zeroing in on its lessons on gender identity and same-sex marriage.

Mr. Ford has also threatened to discipline educators who defy his government’s orders to use the interim sex-ed curriculum, and he launched a platform that critics have called a "snitch line" for parents to anonymously report concerns about what is being taught in classroom. The information would be shared with the Ontario College of Teachers, which licenses, regulates and disciplines educators. Complaints on how the curriculum is taught have typically been referred to the school board employing the teacher, and it is unusual for the ministry to get involved.

Kayla Iafelice, a spokeswoman for Education Minister Lisa Thompson, said on Tuesday that, “as this matter is before the courts, it would be inappropriate to comment on this issue [ETFO’s legal challenge].”

Ms. Thompson issued a statement on Tuesday welcoming students back to school. She has been unavailable for comment on the changes her government has announced, including a broad public consultation on rewriting the sex-ed curriculum as well as discussions about math scores, standardized testing and banning cellphones in schools.

Ontario’s elementary teachers are only the latest group to take the Ford government to court. Slightly more than two months after Mr. Ford’s Progressive Conservatives took office, Ontario's government now faces a number of legal confrontations.

Mr. Ford’s Tories have already fought and lost a legal fight with Tesla after the government tried to treat the car maker differently from its rivals during the winding down of an electric vehicle rebate program. The province’s move to end cap-and-trade and cancel a nearly finished wind-turbine project is expected to spawn future lawsuits.

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Provincial lawyers are also in court defending the government’s last-minute move to cut Toronto’s council nearly in half, a decision made at Queen’s Park after campaigning and fundraising by municipal candidates had already began. Along with the City of Toronto, a number of candidates and local groups have launched lawsuits.

Ontarians enrolled in a basic-income pilot started by Mr. Ford’s predecessor, Kathleen Wynne, have started a class-action lawsuit after the Tory government moved to end the program prematurely. Mr. Ford had promised not to end the pilot while campaigning.

The court challenge filed by the teachers on Tuesday is another attempt to find a legal mechanism to stop the Ford government from dropping the 2015 curriculum. A complaint has been filed to a human-rights tribunal under Ontario's Human Rights Code, and the Canadian Civil Liberties Association (CCLA) has sought an injunction to keep the 2015 sex-ed curriculum until a new one can be developed through public consultation.

Lawyer Howard Goldblatt, who is representing ETFO, said the union will likely co-ordinate with CCLA's application.

"The government has intentionally restricted the expressive freedom of teachers, prohibiting them from fulfilling their role as educators and preventing them from communicating accurate information, critical to student health, safety, development and participation in a modern, diverse and pluralistic society," Mr. Goldblatt said.

Mr. Hammond said the "snitch line" undermines the professionalism of teachers and creates a culture of fear in schools. "It is the role of government to build the system in a positive way, not to toss it into chaos," he said.

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The sex-ed component of the health and physical-education curriculum is usually discussed in classrooms in the spring.