Parents at one of the country's top state schools are being sent bills for thousands of pounds in "fees".
King David school in Manchester has been accused of bullying parents into paying pound;1,230 a year for their children's education. It says that it has to compete with independent schools.
Although the contributions by law must be voluntary, parents are sent "invoices" with a running total of how much they owe.
Some parents claim they have received letters, telephone calls, personal visits and humiliating inquiries into their financial circumstances if they fail to pay.
Ester Lyndley, a single mother, said she has been told she owes nearly Pounds 6,000 to the school for her son, Joshua, and daughter, Chanel. She said: "They shouldn't blackmail parents and hold their children hostage like this."
Her solicitor, James Wilson, said that he would challenge the school's practices in court unless a complaint to the Department for Education and Skills succeeded.
Another parent, who asked not to be named, said she had been told she should cancel her satellite TV subscription to pay the contributions.
As voluntary-aided schools, King David primary and secondary are given about pound;2,400 per pupil by the Government, but have to find 10 per cent of capital costs. They also have to fund Jewish studies.
The high school was the third best comprehensive in the country in last year's GCSE league table.
Joshua Rowe, chairman of governors, said they had just spent pound;9 million on buildings and they also had very high security costs.
The wording on the requests for cash, which say in the small print that contributions are voluntary, was approved by the local authority and the DfES, he said. The word "invoice" has been removed.
"It's a great privilege that the state gives to say that you can set up these partnerships, but by definition that means we have to contribute something," he said.
He said teachers would not know which children's parents had paid and which had not, and said governors made only one reminder phone call a year.
Many schools ask for voluntary contributions or ask parents to sign covenants, but the sums involved are usually smaller than those at King David.
The London Oratory school in west London, where Prime Minister Tony Blair sent his eldest sons, drew criticism after it asked parents to pay pound;35 a month.
Margaret Morrissey, spokesperson for the National Confederation of Parent Teacher Associations, said schools were increasingly using high-pressure tactics to secure parental contributions.
Parents gave over pound;200m a year for basics such as books and salaries, The TES revealed last year.
Ms Morrissey said: "A lot of parents choose state education because their finances mean they can't do anything else. They don't expect to pay by the back door."