The exclusive Anglican boys' school announced on Tuesday it was casting off 87 years of single-sex history to take girls in years 3 and 4 next year, and into years 7 and 11 from 2017.

Principal Justin Garrick said the aim was for all year groups to contain female students from 2020.

Dr Garrick said he understood there would be some resistance from some parents who wanted their sons to attend an all-boys school – and indeed the news created considerable debate across Canberra on Tuesday about the pros and cons of single-sex education.

But Dr Garrick said there had been strong parental desire for the change to take place and Canberra Grammar needed to prepare its students to live and work in a world where women played an equal role.

"Becoming coeducational is an extension of normality," Dr Garrick said. "I think it is an important step to take for the benefit of our boys and also the girls for whom we want to open up our education facilities."

He did not believe there was conclusive evidence that girls or boys benefited purely from a single-sex environment.

"Every single student should be taught as an individual."

But the principal of Canberra Girls Grammar School, Anne Coutts, said her school would continue to specialise in the education of girls.

"We absolutely reaffirm our commitment to remain a single-sex school. We are experts in girls' education and, indeed, we have produced the top students in year 12 across Canberra for the past four years running."

In a signal that the formerly close ties between the complementary southside Anglican schools may now be strained, Ms Coutts said the Canberra Grammar decision was "interesting" and had not been taken in consultation with the girls' school.

The move may force the girls' school to consider its reciprocal enrolment agreement with Canberra Grammar, and indeed could potentially lead to a reassessment of its practice of enrolling boys up to Year 2.

Both schools began the arrangement more than 40 years ago, when they allowed coeducational intakes from pre-school to year 2.

Boys at the girls' school are guaranteed a place at Canberra Grammar at year 3, while girls at the boys' school are guaranteed a place at Canberra Girls Grammar at year 3.

The decision to co-ed will mean Canberra Grammar will no longer automatically transfer its girls over, although parents will ultimately decide where they send their daughters.

Dr Garrick said he was "acutely aware that for many families that have sons and daughters, they deeply wish their children to be educated at one, excellent, school."

Dr Garrick noted "modern life is complicated enough without the requirement to place siblings of different ages and genders in multiple schools with different drop-offs and pick-up arrangements, different policies, procedures and communication systems, different dates, events and curriculum structures to negotiate".

The school would keep its name and introduce changes to the uniform, which would be phased in from 2016.

Dr Garrick did not believe the school had an aggressively male culture and said every effort would be made to ensure new girls had "full citizenship" at the school from sporting, to cultural and academic pursuits.

The school would invest in new toilet and change rooms facilities in preparation, as well as taking expressions of interest in coeducational boarding from 2018.

With current enrolments at a record high of 1700, Dr Garrick said the move was not an attempt to double the student population. But extending enrolments to girls would "underpin the school's prosperity into the future".

"We face potential competition from the advent of new schools and from existing schools with the capacity to expand. It is virtually inevitable that the need to enlarge our enrolment pool through co-education will come upon us at some point. We believe it is right to embrace that reality now, on our terms, not later as a consequence of decisions made by others."

"We have spoken with Canberra Girls Grammar about our decision and we hope to continue having a close and positive relationship with the school."