A civil-liberties group is asking a court to stop Ontario’s Progressive Conservative government from throwing out a contentious sex-education curriculum, calling its planned replacement with a 20-year-old document an act of illegal discrimination against the LGBTQ community.

The Canadian Civil Liberties Association is seeking a court order as early in the school year as possible to maintain the previous sex-ed curriculum until a new one can be developed through public consultation. Its filing in Superior Court on Thursday comes a day after the provincial government directed public-school boards to use a “revised, interim curriculum,” based on a 1998 document on sexual health, this school year. The curriculum it is replacing, implemented in 2015 by a Liberal government, includes sections on gender identity, sexual orientation and consent not found in the 1998 curriculum.

The government has said it will hold public discussions while it develops a new approach to sex education. But Michael Bryant, executive director of the Canadian Civil Liberties Association, called the government’s approach a “dog-whistle of homophobia dressed up as a consultation.”

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“The government has just ripped out all of the material in the sex-ed program other than the heterosexual content,” Mr. Bryant, a former Liberal attorney-general , told reporters.

Becky McFarlane, a queer mother of a 10-year-old daughter entering Grade 6 at a Toronto public school, has also joined the civil-liberties group’s challenge. She was not available for comment. But Mr. Bryant said the government’s revised curriculum treats same-sex families such as hers as if they don’t exist. “It’s as if they are the others, the unspoken others.”

Kayla Iafelice, spokeswoman for Education Minister Lisa Thompson, said Thursday the government expects teachers to use the interim curriculum, and will consult with parents and educators on a revised document. She did not otherwise respond to a question about the legal challenge.

The court challenge is the second attempt to find a legal mechanism to stop the Ford government from dropping the 2015 curriculum. Representing an 11-year-old transgender girl, Toronto lawyer Marcus McCann filed a complaint this month to a human-rights tribunal under Ontario’s Human Rights Code. The two legal challenges are part of a co-ordinated strategy, both Mr. McCann and Mr. Bryant said in interviews.

“The 2015 curriculum included information about her identity and her body,” Mr. McCann said in an e-mail to The Globe and Mail, referring to the 11-year-old, “and that’s now been removed. That’s a breach of the code, because her non-trans peers will still get access to information about their identities and their bodies."

The civil-liberties association says it will rely on the precedent set in June in the Trinity Western University case, involving a private Christian institution attempting to start a law school while insisting students sign a covenant barring sexual activity outside of heterosexual marriage. The Supreme Court of Canada said law societies are entitled to deny the school accreditation, because it had not given proportionate weight to the Charter rights of gay students.

On Wednesday, Premier Doug Ford warned teachers not to use the 2015 curriculum. The government also announced a platform for parents to report concerns anonymously over what is being taught in classrooms. The information would be shared with the Ontario College of Teachers (OCT), which licenses, regulates and disciplines educators.

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

Andrew Campbell, a Grade 5 teacher in Brantford, Ont., who has taught for more than two decades, said he would look for other places in the school day where he could engage students in discussions that were part of the now-scrapped version of the sex-ed curriculum.

“We’ll deliver the curriculum as it is written and then we’ll find other places to make sure our schools are welcome and open to all different kinds of families and all students,” Mr. Campbell said.

A.J. Adams, who is a supply teacher at two school boards and has been teaching for three years, said that if a student were to ask a question about content not covered in the PCs' interim program, she would still provide the relevant information. She discussed consent and online safety with Grade 7 and 8 students when the 2015 curriculum was in place.

She added that she understands that her union will defend her in teaching the 2015 content.

Kimberly Rastin, who will be teaching a Grade 4-5 class this year within the London District Catholic School Board, said she was frustrated by the Ford government’s move to revert to a 20-year-old curriculum.

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"I don’t see how I’m supposed to talk about family development without acknowledging that there are all kinds of families out there, there are all kinds of families represented in the Bible,” she said.

Ms. Rastin, who has been teaching for 18 years, said she and other teachers are nervous about the situation.

“I’m not looking to get fired. But on the other hand, I’m also not looking to be a lesser teacher than I was last year and do lesser service to them [the students] than I did last year. It is my job, it is my responsibility, to prepare these children for the world in which they will live,” she said.