The terms of Private Finance Initiative contracts are routinely covered by non-disclosure agreements - but some headteachers are now speaking out as they face spiralling costs.
School leaders told the BBC they are spending tens of thousands of pounds more a year to meet the rising costs of the contracts with private firms.
The PFI schools are locked into contracts of up to 30 years, in which charges rise more than at other schools.
She said: “If there was more openness, the publicity would shock many citizens and taxpayers, and that might push the companies to think again and make sure they're not wringing every last penny out of our school system.”
Headteacher David Potter said nearly 20 per cent of his school’s budgetis now spent on meeting “frustrating” terms of the PFI contract, which means he has to spend less on staff - four members of classroom staff have not been replaced since 2020.
All of the maintenance, catering and cleaning for the school – Middlefield Primary in Speke, Liverpool - is included in the PFI contract which will cost more than £470,000 this year - a rise of more than £151,000 since 2021.
One of the “rigid” details in the contract is the stipulation that the playing field grass must not grow more than 2.5cm high.
Mr Potter told the BBC: “We're in the middle of February, the ground's a bit waterlogged, so we won't use this space very much at this time of year.
“But come rain or shine every week, the grounds maintenance team come out and they cut this field. We should have the freedom to say, actually, we think we can do without."
Maintaining the grounds costs the school around £30,000 a year. The contract does not allow him to shop around for better prices from other suppliers.
The PFI company told the BBC it would be willing to renegotiate for the grass to grow to 5cm, but Liverpool City Council said the legal costs would outweigh the benefits.
Ten other PFI primary schools in Liverpool provided figures to the BBC showing similar price rises.
More than 900 schools in England were built through PFI contracts, with the first opening in 1999. The initiative was scrapped in 2018.
These state schools are owned and maintained by the private sector during the contract, until taxpayers' money has repaid the debt and the buildings go into state ownership.
PFI costs go up by the Retail Price Index, a typically higher measure of inflation.
Some headteachers told the BBC they had been advised against speaking publicly about the pressures PFI costs are causing, because of non-disclosure agreements that are built into the contracts.
Last week, Stoke-on-Trent City Council held a meeting in private with the 88 PFI schools in the city which were warned they face "double digit"percentage increases in their PFI costs within weeks.
The Department for Education said it was increasing support for schools in PFI contracts by 10.4 per cent in the coming financial year.
Speaking on behalf of PFI investors, Lord John Hutton said price comparisons were made regularly, but school budgets had not kept up with inflation over the lifetime of PFI contracts.