TORONTO -- A union representing Ontario's elementary school teachers is calling on its members to continue to teach a modernized sex-ed curriculum despite the provincial government's decision to revert back to a 20-year-old lesson plan.

The Elementary Teachers Federation of Ontario "strongly denounced" the Progressive Conservative government's move to repeal the curriculum updated by their Liberal predecessors in 2015, which taught concepts such as consent, LGBTQ rights and online safety.

The Tories have said teachers will use a version of the curriculum developed in 1998 as the government carries out public consultations for a new document.

ETFO President Sam Hammond criticized the approach and said the union is advising all members to "exercise their professional judgement" on sex-ed matters when school resumes in three weeks.

"The government's decision to revert to the 1998 Health and Physical Education Curriculum while initiating further consultation is irresponsible, discriminatory and jeopardizes the safety of the students that we teach," Hammond said in a statement issued Monday as the union held its annual meeting.

"Teachers will not be muzzled by a government whose political agenda takes precedence over the protection and education of their students."

Hammond said abandoning the updated curriculum in favour of an older one risks putting student safety and human rights at risk, adding that following the government's edict in this situation creates a "direct conflict with teachers' fundamental responsibilities and obligations towards their students."

Calling the government approach "reckless," Hammond said the union would defend all teachers who continue to follow the 2015 curriculum. ETFO also said it would intervene in cases regarding the curriculum that come before the Human Rights Tribunal of Ontario, such as one a group of parents have announced they plan to file.

Six families from across the province have said they will mount a human rights challenge against the government decision on the grounds that reinstating the curriculum discriminates against LGBTQ students. The lead complainant in that case is an 11-year-old transgender student entering Grade 6 in the fall.

Lawyers arguing the case said a curriculum that acknowledged the existence of LGBTQ students not only sent a message of acceptance, but created a healthier classroom atmosphere and gave teachers the tools necessary to guide discussions and offer support to all students.

Challenges to the recent government decision have come from several other quarters as well.

Last week, NDP Leader Andrea Horwath delivered a petition to the legislature signed by nearly 1,800 doctors, nurses, social workers and other health-care providers. The document said repealing the modernized curriculum puts children's health at risk by denying them key information about concepts such as consent, safety and inclusivity.

The updated sex-ed curriculum included warnings about online bullying and sexting, but opponents, especially social conservatives, objected to the parts of the plan addressing same-sex marriage, gender identity and masturbation.

Premier Doug Ford accused the Liberals during the spring election campaign of introducing a sex-ed curriculum based on ideology, saying they had turned schools into social laboratories and kids into test subjects. He promised a Tory government would replace the curriculum with a teaching plan that was "age-appropriate."

More than two dozen school boards in the province have expressed concerns about the government's plan, with some saying teachers will continue to discuss the issues included in the modernized curriculum in their classrooms.

Hammond said the union recognized the boards' "strong response" and called on others to join in supporting teachers who eschew the 1998 curriculum.

The Ministry of Education did not immediately respond to request for comment on ETFO's statement.