Primary schools in England have almost pound;750 million in their bank accounts, with average surpluses of around pound;50,000 each.
The figures will embarrass heads - meeting this weekend for the first time since withdrawing from the workforce deal - who say they cannot afford the scheme.
The most recent figures from the Department for Education and Skills show that in 20034, 90.6 per cent of primaries had an average surplus of pound;49,154; for the rest the deficit was pound;20,795.
The average savings could fund two supply staff for a year. The biggest surplus was at Harpur Mount primary, in Manchester, with pound;697,362.
The worst off was Gascoigne primary, in Essex, which was pound;385,925 in the red.
By law, primary teachers must have 10 per cent of their teaching time away from the classroom from September for planning, preparation and assessment.
But many schools are a long way from putting the deal into practice.
It will be a major issue at this weekend's annual conference of the National Association of Head Teachers, in Telford. Its members face legal action from the National Association of Schoolmasters Union of Women Teachers if the reforms are not carried out.
Chris Keates, general secretary of the NASUWT, said: "Claims of underfunding from some heads can never be taken seriously while there are huge amounts of money sitting in balances. This has been going on for years and is a flaw in the system. Parents should demand to know from the Government why money for educating their children is not being spent for that purpose."
David Hart, general secretary of the NAHT, said: "Schools will have to explain why money carried forward has not been used for remodelling when this is the number one priority."
Prime Minister Tony Blair promised this week that he and Education Secretary Ruth Kelly would look at problems over PPA time. On a visit to Lilian Baylis school, south London, Mr Blair said: "We need to make sure it's properly done. It's a question of how we allocate the money we've got in the education system."
He was responding to questions from Chris Luck, head of Eastfield primary, north London, who said he could not guarantee PPA unless his salary budget increased by a 10th. But his school had a surplus of some pound;50,000 - enough to fund two new teachers.
A Labour party spokesman said: "We have put substantial money into making this possible, and most schools are getting on with making it work for teachers, support staff and pupils. We will make sure teachers get the time they are entitled to."
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