A head teacher hired a private detective to check the addresses of parents suspected of lying to get their children into his school.
The school has more than 1,000 pupils including 100 boarders
The school, St George's in Harpenden, Hertfordshire, offers 160 places based on church membership and residency.
But some parents have tried to bypass the entry criteria by renting property.
Head teacher Norman Hoare said he hired the detective to check one address for three days in attempt to find out if the application was genuine.
"It's come to light for the last five or six years now that there have been incidents of parents seeking to leapfrog our criteria by renting property very close to the school and therefore qualifying on residence," he said.
On three occasions in the last five years Mr Hoare and his deputy had visited addresses supplied by parents.
They sat outside in cars to check if the family went in and out and rang the doorbell at times when they would be expected to be in.
When challenged, the parents in those cases withdrew their applications, said Mr Hoare.
The voluntary-aided faith school has control of admissions for Year 7 pupils but is always oversubscribed and uses residency either in Harpenden or surrounding villages as one entry criterion.
In one "particularly difficult case" the school hired a private detective and challenged the family on the basis of his report, but the parents insisted they were genuine, said Mr Hoare.
"We felt so strongly at that stage that we took the unusual step, but I would take it again, to hire a private detective for three days."
He added: "Parents all want the best for their children and they will go to any lengths and I can't be critical of those parents.
"I don't like it happening because of the extra work and it can lead to unpleasantness."