Newington College in Sydney's inner west announced its intention in November to shift to co-education across its kindergarten-to-year 12 program.
The school, which charges fees of up to $42,200 per year, has exclusively taught boys since it was founded in 1863.
Newington College is facing a backlash over plans to become co-educational. (Bianca De Marchi/AAP PHOTOS)
But the decision, which the school council said was made to boost diversity and "life-readiness" at the college, has led to a fierce backlash among some parents and former students.
About 25 people protested outside the school as students returned to classes on Wednesday after the summer holidays.
Many complained that the move to co-education represented an unacceptable change to the school's culture.
"Old Newingtonian" Tony Retsos said he intended to pay for his first-born grandson to attend the school.
"There are current parents that signed up on the basis that it's an elite boys' single-sex school," he told AAP.
"That's what it's been for 160 years."
Fellow alumni Robert Orr agreed, saying the decision would mean there was one fewer boys-only school for Sydney parents to choose from.
"I suspect it's for virtue-signalling, woke-type principles, which I'm dead against," he said.
The school's council chairman, Tony McDonald, previously said the switch was intended to promote inclusiveness among students and the decision was made after feedback from students, parents, staff and alumni.
The staged shift to co-education will start in the junior school in 2026 and for high school students from 2028, with the college fully co-educational by 2033.
One parent who attended the protest but declined to give his name said he expected the change would lead to worse educational outcomes.
"You'd be mad to send a girl to a boys' school that's going to have 200 boys and 100 girls," he said.
"I don't think it's going to benefit the boys either."
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