Joe Morrisey, who advocates an independently funded year 7 to 12 school to be run on a campus-sharing arrangement at Mount Carmel from next year, accused Archbishop Christopher Prowse of putting a "monument" ahead of education, in an open letter he sent to The Canberra Times.

"Obviously a monument is more important than children's education," he said. "You are prepared to budget $35 million for a building in Canberra but couldn't find $250,000 to cover a doubtful deficit for the school."

Church officials reject the claim, saying it is wrong to link the school closure to the redevelopment project.

Archdiocesan financial administrator Helen Delahunty also said the Church had not "shut the door" on re-opening the Yass secondary school if sufficient pupil numbers could be guaranteed.

Enrolments at the secondary school, which accepts students from years 7 to 10, have fallen by more than half, from 150 in 2004 to just 61 at the start of this year. It will cease to operate at the end of this year, with all permanent staff to be offered redeployment.

There are no plans to close the Mount Carmel Primary School, which has more than 300 pupils.

Ms Delahunty said an offer by Archbishop Prowse to form a joint working party headed by an independent chair to investigate the reopening of the school and the addition of years 11 and 12 was still on the table.

"There is a deep desire for a year 7 to 12 Catholic secondary school in the Yass region, rather than simply a year 7 to 10 model operative in Yass since 1993 when the Mercy Sisters left the school," Archbishop Prowse wrote in a letter to parishioners outlining the proposal in June.

Ms Delahunty said the Mount Carmel College steering committee instead chose to focus on the establishment of an independently funded school to be run by Mount Carmel College Ltd.

That plan was rejected by the archbishop this month on the grounds campus-sharing was "not viable".

Ms Delahunty said the Yass school and the cathedral precinct redevelopment, which will centralise offices and create a complex of independent living units, are under different funding and management streams.

"St Christopher's cannot access any funds that have been earmarked for education," she said.

"Education funding is quite separate and much of it comes from the state and federal governments. We are held accountable for how it is spent."

Mr Morrisey blames the archbishop for the failure to follow through on the working party.

"You did nothing about the working party you promised – so how can we trust your word?" he wrote.

Ms Delahunty said the decision not to operate the high school in 2015 had been a hard one.

"I can appreciate people are hurt and disappointed. Shutting down schools is never good but in this instance it has validity. The Yass community needs to understand the [working party] option is still there if they are willing to work with us."