TORONTO -- Ontario is increasing class sizes for Grades 4 to 12 in an effort to cut costs, a move critics say will hurt student learning and result in thousands of teaching jobs lost across the province.

The move is part of education reforms announced by the Progressive Conservative government Friday that also include introducing a sex-ed curriculum that returns to teaching gender identity and consent after a modernized lesson plan was scrapped, revamping the math curriculum and making students take more online courses.

"Our plan will modernize the classroom, will protect the future of our education system, and will ensure Ontario's students acquire the skills they need to build successful lives, families and businesses," said Education Minister Lisa Thompson.

The government has been consulting for months on several education issues, including class sizes, teacher hiring practices and sex-ed.

On Friday, the province said average high school class sizes will increase by six students -- from 22 to 28. Thompson said the change would be phased in over four years and noted that Ontario high schools currently have one of the lowest student-to-teacher ratios in the country.

Average class sizes for Grades 4 to 8 will increase by one student per classroom -- from a current 23 students to 24. Class sizes for kindergarten through Grade 3 are not changing.

"Not one teacher -- not one -- will lose their job because of our class size strategy," said Thompson, although she could not immediately explain how the government would achieve that goal.

The government said the changes were being made to "better balance student success and system sustainability" but could not immediately say how much the move would save the province.

The head of union representing Ontario's public high school teachers said the change means about 3,600 secondary school teachers will lose their jobs over four years.

"(It's) a loss that cannot possibly be absorbed without a significant impact on student learning and success," said union president Harvey Bischoff, adding that Premier Doug Ford's government has "declared war" on the education system.

"Given the premier's repeated election promises that no jobs will be lost, the government quite simply has no mandate to make the changes they have announced."

Bischoff added that changes to several grants to schools will also meaning funding will drop by $1.4 billion for the education system.

The head of the elementary teachers' union also predicted job losses and said students would suffer.

"Larger classes mean less support for individual students and will disproportionately impact students with special needs," said Elementary Teachers' Federation President Sam Hammond.

Hammond said his union will fight moves to hike class sizes and change hiring practices, noting that those issues are normally the subject of collective bargaining negotiations.

Both unions are set to begin contract negotiations with the government this summer and Hammond said Friday's news sets a "very concerning and negative atmosphere around those talks."

NDP Education critic Marit Stiles said increasing high school class sizes will affect students' ability to learn.

"What we hear constantly from parents is that their children are not getting the one-on-one attention they need in the classroom," she said.

Ontario's former Liberal premier Kathleen Wynne said the changes will result in a $250 million cut in funding to the province's education system and undo improvements made by her government.

"What is clear is that students, who are the ones on the receiving end of these cuts, will be worse off," Wynne said in a statement.

Meanwhile, schools will have a new sex-ed curriculum that will replace a much-criticized interim teaching plan brought in after the Progressive Conservatives took power last year.

The Tories, in a move to cater to social conservatives, scrapped a modernized curriculum brought in by the Liberals that addressed consent, online bullying, sexting, same-sex relationships and gender identity.

The new document will return to teaching those lessons, but in some cases will do so when students are older. Gender identity, for instance, will now be taught in Grade 8 after previously being taught earlier under the Liberals. The curriculum will also include teachings on abstinence and lessons on cannabis.

Parents will still be able to opt out of having their kids exposed to certain topics in the sex-ed class, and the government will issue online modules for those who want guidance on discussing those topics at home. The full curriculum is expected to be released in May and implemented in September.

A new math curriculum will also be phased in over four years. It will focus on basic concepts and skills, with the first changes taking effect this fall.

The current so-called discovery math curriculum, which Ford has railed against, is being scrapped. Discovery math does not focus on memorization of math problems that were traditionally taught in the classroom. Instead, it encourages students to engage in problem-solving that grounds math in its application.

The government said it will also make online courses a larger part of learning starting in 2020-2021. Students will be required to take a minimum of four credits out of 30 through e-learning courses to receive their diploma.

The average e-learning class size will be 35 students.

Ontario high schools will also receive a revised curriculum on First Nations, Metis and Inuit studies, which the province said was developed in collaboration with Indigenous partners.

The government will also ban cellphones in classrooms during instructional time, starting next year, except for when teachers want to use cellphones as part of their lesson, for medical reasons and for students with special needs.