A city prep school dubbed “the 11-plus factory” has fallen foul of advertising standards for over-egging its Kent Test results.

St Christopher’s in Canterbury boasted of a 94% pass rate in last year’s grammar school qualification exams.

But a Freedom of Information request reveals the reality is less impressive – exposing a pass rate of 53% and a number of pupils passing on appeal.

Officers of the Advertising Standards Agency have now taken the £8,400-a-year school to task following complaints from a member of the public.

The school’s promotions on advertising hoardings – including on double-decker buses – and online must now be altered to remove the misleading claim.

Joanne Bartley is chairman of Kent Education Network, a campaign group opposing the 11-plus exam in Kent.

She said: “Because the 11-plus is not transparent, nobody knows who passes and who fails. Results are never published, so schools can easily misrepresent themselves.”

Pupils taking the 11-plus exam must accrue 320 points across three papers, with no single score of less than 106.

According to a Freedom of Information request submitted to Kent County Council by Mrs Bartley, 17 pupils took the 11-plus exam at St Christopher’s in 2015. Nine passed outright, representing a 53% pass rate.

A further six cases were put to head teacher appeal for pupils who had exceeded the 320-point overall target, but had dipped slightly below 106 points in one of their papers.

While not passes, five of the six appeals were successful, and according to the school, a separate case was won on a “parents’ appeal”.

The result was 15 of the 17 pupils winning grammar places, representing an 88% success rate rather than the “over 94% pass rate” as claimed by the school.

Mrs Bartley said: “There’s no one checking, so no one really knows. We don’t know who’s passed and who’s through on appeal.

“Some head teachers are more pushy with appeals than others. And sometimes it’s just a point or two that separates a successful pupil from an unsuccessful one.”

In a response to Mrs Bartley’s complaint, the ASA wrote back: “Once we reviewed the ad in light of your complaint, we thought you had a valid point and instructed the advertiser to amend the ad – they have now agreed to do this.”

The ASA confirmed it had investigated and the school would withdraw its advert.

A spokesman said: “We will not be making a comment on this case until it is due to be published next Wednesday as per our protocol.”