Mr Rattenbury wrote to Mr Barr this month, saying the land should not be taken from Telopea.

"I expressed my concern about the whole deal," he said. "If we boil it down ... this is about replacing the Canberra Services Club after the fire. And it seems that to address what is a fairly clearly defined issue a number of other pieces of public land have been drawn into a rather complicated scenario - which is bringing a level of upheaval that seems out of proportion to the initial problem."

The government has taken the Telopea Park tennis courts off the school, planning to sell the land for a child care centre, so it can relocate the Manuka Occasional Child Care Association from its Manuka site, so it can give the Manuka site to the services club. The move is strongly opposed by the school community.

"The club is the issue we're trying to fix here and whole lot of other people have been drawn into what's become a very complicated response," Mr Rattenbury said.

While the government has refused to budge so far, Mr Rattenbury's intervention is a very strong indication that the school will get a win and be able to keep its tennis courts.

Mr Barr relies on Mr Rattenbury's vote for a majority in the Assembly, and if he doesn't resolve the Telopea impasse, he leaves open the possibility of being forced to act by a vote of the Assembly. Mr Rattenbury has not made this threat and did not say whether he would go so far to force a government backdown, but said he wanted it resolved outside the Assembly.

Association president Paul Haesler was cautious, saying he was "greatly heartened" by Mr Rattenbury's intervention, but was still waiting for Mr Barr's response.

"We are not taking anything for granted and will continue our campaign until the tennis courts haves been returned," he said.

"We haven't won yet, there's still a battle ahead of us, but I'm more confident now than I was a couple a months ago."

Also this week, the Education Directorate released calculations of space available to Canberra's high school students, data aimed at giving the lie to claims by the Telopea Parents and Citizens Association that its students are among the most cramped in the city.

The Telopea Park School calculated space per student in May, arguing that it was already short of space compared with others.

The association's figures suggested Telopea was the most crowded high school in the city (including K-10 and P-10 schools), with 167 students per hectare, compared with the next highest, Amaroo, at 65 students per hectare.

Education officials rejected the figures at ACT Assembly estimates hearings this week, releasing their own calculations.

They show Amaroo and Harrison in Gungahlin are the most cramped in the territory, each with more than 200 students per hectare. Next comes Lyneham High School, then Telopea in fourth place.

Telopea Park has between 170 and 178 students per hectare, depending on whether the tennis courts are counted; figures broadly in line with the school's calculations. Caroline Chisholm and Wanniassa, both in Tuggeranong, are the most roomy school campuses in Canberra, with more than double the space of the most cramped.

Mr Haesler said the school had repeatedly requested the figures, including from Education Minister Joy Burch when she told him his figures were wrong, but she had refused to release them til now.

One of the big differences between his and the government figures was student numbers, with the government using capacity and the association taking its numbers from the 2014 myschools website. The association had also included public ovals close to schools, which schools used for sport, including at Amaroo and Harrison, which both used adjacent playing fields, and Namadgi, which was adjacent to Kambah fields, he said.

"While we appreciate the government supplying them now ... the fact that they refused to make them available to us until asked by a journalist highlights the lack of respect towards the school community the government has exhibited through all its handling of this issue," he said.