A headmaster has employed a private detective to spy on parents suspected of lying about their address to obtain places for their children at his school.

Norman Hoare, head of the oversubscribed St George’s School in Harpenden, Hertfordshire, has also resorted to going on early-morning and late-night stake-outs of flats and houses of potential applicants to see if they really live where they say they do.

His hard-line approach suggests that as parents adopt extreme cunning to get their child into a good state school, wily heads are fighting back.

Mr Hoare said he made no secret of his determination to check out the veracity of application forms to St George’s, one of the top performing schools in Hertfordshire, with 95 per cent pass rate for pupils gaining five good GCSEs.

He had become something of an expert, he added, in spotting the telltale signs of a fraudulent application, such as the same address appearing on the forms of different families year after year.

Mr Hoare said: “We believe we have been justified in taking unusual measures. We have staked out addresses in Harpenden which we know to be small flats that are probably not the kind of permanent residence you would expect for some of the children who apply. We also hired a private detective to check out an address for two to three days.”

He also makes spot checks on parents’ addresses during the middle of the day. “When the person who opens the door is aged 80, you can bet it’s the grandparent and the family doesn’t live there,” he said.

Mr Hoare said that he had caught out two families through spot checks. One withdrew their application when confronted. The other took their case to appeal and won.

While parents are required by the school to provide utility bills as proof of address, Mr Hoare said that these were not reliable as they could be easily obtained by parents renting a property for a short period.

Hertfordshire County Council has tough rules about school places fraud. Last December the High Court ruled a 12-year-old boy should lose his place at a St Albans school because his parents failed to provide proof of his permanent address. But St George’s, as a voluntary aided school, manages its own admissions and takes responsibility for checking the legitimacy of applications.

St George’s takes in 20 boarders and 160 day pupils each year. Boarding places are so oversubscribed that they are usually fully allocated more than a year in advance. All the cheating attempts relate to the 160 places for day pupils, which are between three and four times oversubscribed.

St George’s is not alone. Councils are finding instances of parents giving false addresses or tactically renting for a month or two in catchment areas, to enrol their child in a popular school before moving back to a home outside the catchment area. Frequently they are exposed by tip-offs from the playground.

A recent ICM poll for Teachers TV found that nearly half of parents would consider giving a false address inside a secondary’s catchment area or even pretend to be religious.

No admissions system is fool-proof. Some state schools use an entrance examination to divide children into ability bands before selecting a proportion of pupils from each band.

Professor Alan Smithers, of the University of Buckingham, said it was not beyond some parents to try to cheat this method. He suggested: “For high-performing schools, there are likely to be more pupils in the top band, so a cunning parent might suggest that their child do badly in the test so they fall into the bottom band, where they have a better chance of being selected.”

In North Yorkshire, the council’s Selby office has an annual problem with parents determined to gain places at Tadcaster Grammar School, a nonselective state school with a superb reputation.

The council said between five and ten families were caught cheating each year. “Most will discreetly withdraw at the application stage when confronted,” it said.

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