One of them is Mina Sakai, who is considering what impact the changes will have on her 12-year-old son.

"For us it is like making a financial commitment to a dream car, only for the factory to suddenly tell you that model has been discontinued and you are getting a different car. We feel upset because we were not told about the change, and we were not asked what our opinion was."

Ms Sakai and her husband both went through single-sex schools and chose Canberra Grammar specifically for its boys-only character and old boys' network. They registered for enrolment at CGS soon after their son was born 12 years ago because it was an all-boys school.

The family lives in Kaleen, where the co-educational Anglican Radford College was a much more convenient – not to mention cheaper – option. Canberra Grammar fees for year 6 are $17,070 while Radford College charges $11,200.

"It is a 10-minute bike ride away but we chose Grammar because we definitely wanted a single-sex school for our son on top of quality academic, music and sporting options and all the traditions that the school offered."

Moving their son to Radford could be an option if other friends of her son's left the school. "We will wait and see."

Ms Sakai is hoping to gather a group of similarly-angered parents to appeal to the school to change its mind or reconsider the proposed timeline.

Since announcing the move to take girls after 87 years as an exclusive Anglican boy's school, principal Justin Garrick acknowledged that not everyone was happy about the decision.

He had received a number of emails and phone calls from distressed parents – particularly on Tuesday afternoon - but he said he had received a greater number of contacts which offered "enthusiastic support for the change".

The school had also received more than 200 enrolment inquiries, which Dr Garrick said "speaks volumes about the desire for our school to embrace the co-educational model and faith people have in us to offer a fantastic education for their sons and daughters".

Dr Garrick said: "I understand it is a big change but as people have time to process some of their thoughts and feelings I am confident there will be understanding about the strategic reasons behind our decision."

The strategic reasons included "ensuring the school looks forward and continues to provide an educational experience that matches the realities of modern society, family life and the workforce".

On the issue of consultation, Dr Garrick said he did not believe the school community had been blindsided by the announcement.

"Co-education has been discussed for years and had been a part of our community surveys as far back as 2008. It is one of the issues that has been subject to both formal and informal discussion and is certainly a matter that is raised every open day I have been at. It has also been under consideration for some years at a board level."

Even the school's Parents and Friend's Association is believed to have expressed its concern that the decision was taken at a board level without consultation with the parent body.

But Dr Garrick said it would not have been possible to run any sort of plebiscite on the issue.

He said that "open debate would have entailed the strongest voices carrying the day and others disappointed whatever the outcome … The reality is if you open up to a full discussion you destabilise the community and you still end up with some people unhappy or happy after a long period of uncertainty in a competitive environment.

"We felt we needed to have a serious consideration of the issue and then make a decision that was clear and direct."

He said he would make time for any parents who wished to speak with him.

Meanwhile, Marist College headmaster Richard Sidorko confirmed that there had been a greater number of enrolment inquiries on Wednesday than usual.

Mr Sidorko did not want to comment on specific enrolment matters, but confirmed school's commitment to single-sex education for boys.