The future of a voluntary grammar school in Londonderry could be under threat unless it repays £821,000 to the Department of Education.
Foyle College moved to a new site in the Waterside area of the city three years ago, but the loan used to foot the bill for the building has yet to be repaid.
The school has sent a letter, obtained by the BBC, to former pupils, asking for help in covering the debt.
Foyle College said that while the department had offered to defer the repayment, making changes to the terms of the loan would be problematic.
The school spent £2.4m on additional facilities in the new build "beyond what public funding could provide" but denied knowingly overspending.
Gavin Killeen, the chair of the board of governors, explained: "The board of governors quite a while ago decided that we were not going to move very often. You’re going to do it once every two or three generations.
"We wanted to make sure that we were moving to the best possible school. We were very happy to enhance the budget that the department had agreed to fund.
"We were able to add extra facilities, extra rugby pitches, extra hockey pitches. We have a sports hall that can accommodate five badminton courts."
He added that past pupils previously made "very significant" donations towards meeting the initial cost of the building.
"We raised well in excess of £1m," Mr Killeen told BBC Radio Foyle.
"You don’t know exactly what you have until everything is finalised, so it’s only really over the last eight months that we know the final position.
"If we weren’t able to repay the department, our future, like any other business or any other organisation, would come into question."
The school added it was confident the money would be repaid.
Voluntary grammar schools are funded by the department but manage their own budgets.
A Department of Education spokesperson said: “As per normal procedures with schools, the Department is continuing to work with Foyle College to secure repayment of funds related to their move to the new site at Limavady Road.”
In 2013, Bloomfield Collegiate in Belfast became the first grammar school in Northern Ireland to give up its voluntary status because of a financial deficit.