The National Association of Schoolmasters Union of Women Teachers survey shows primary school balances are up by 12.43 per cent on last year, despite the fact that the last teachers' pay increase has not been funded.
More than Pounds 500 million has not been spent from the 19945 budget.
The average underspend in primary schools is Pounds 23,500, which amounts to 7.45 per cent of the money allocated. This, says the union, is equivalent to a full-time, fully qualified teacher.
The average secondary school has 3.89 per cent of its budget underspent, but the overall secondary underspend has gone down by more than 9 per cent.
Such balances are politically sensitive because the Government last year told the School Teachers' Review Body to take them into account when setting the pay settlement.
In reaching its recommendation for a 2.7 per cent rise, however, the STRB pointed out the enormous variation in school budgets, making it difficult to use them to reach decisions on pay.
Brian Clegg, NASUWT assistant secretary (salaries and pensions), said: "Children are being cheated by these shortfalls. This money has been allocated for education and should be spent. We accept governors may want a contingency fund, but can't see how they can justify keeping back more than 1.5 per cent."
He also condemned special schools, which have on average a Pounds 29,500 balance - 5.36 per cent of their budgets: "It is scandalous that schools for children with special needs are not making sure every penny possible is spent on their pupils."
These figures, based on more than 500 schools, are the most up to date in a survey that will eventually include 15,000.
The findings have been sent to the School Teachers' Review Body, which will report at the end of the month.
The union has been campaigning to make it compulsory for schools to declare the reason for keeping money back when publishing their Section 42 budget statements.
"Some local education authorities are beginning to ask such questions, " said Mr Clegg. "And we have discovered that schools are often holding on to the money because they plan to put up a building.
"However, this money is totally separate from the capital budget and should not be spent this way.".
The situation does vary enormously. The survey includes one secondary school with an underspend of more than Pounds 1 million, and in Manchester there is an average deficit of only Pounds 569 in primaries but Pounds 33,192 in secondaries.
Walter Ulrich, spokesman for the National Association of Governors and Managers, said his members were perfectly within their rights to keep a balance.
"Local management allows schools to carry over money from one year to the next and governors are often quite rightly saving up for something they need. Capping means that local education authorities cannot always spend what is required on buildings and other capital projects, therefore schools have to finance them themselves," he said.