It could be seen as a blow for either grammar school or academy evangelists. It also shows the uncompromising nature of England’s school accountability system.

Today it emerged that Chatham Grammar School for Boys - where an impressive 95 per cent of pupils gained five good GCSEs this year - has been placed into special measures.

The Kent school, which became a converter academy in 2011, is thought to be only the second grammar to be rated as inadequate.

A blistering Ofsted report said leadership of the school was inadequate because achievement, especially in English, had not improved quickly enough since the last inspection.

Despite Chatham pupils’ good exam results, pupils of comparable ability in other schools were doing far better in some subjects, inspectors found.

They also said there was a “stark contrast” between pupil behaviour outside lessons, which was good, and poor behaviour during lessons.

Showing that you don’t have to be in a “bog standard comp” to experience bad lessons, inspectors said leaders had failed to eradicate “weak and inadequate” teaching.

“Teachers do not consistently plan lessons that challenge students and extend their thinking or give students detailed verbal and written feedback on how to improve their work,” the report said.

Ofsted warned that there was not enough accurate checking of students’ progress, especially for those from poorer backgrounds and youngsters taking GCSEs and A-levels early.

It concluded that pupils’ behaviour in lessons had worsened, saying youngsters “lose interest and motivation when teaching is not good enough.”

The school’s head David Marshall left this summer - after contesting the Ofsted findings. It has now been placed under the auspices of senior leaders at the Rochester Grammar School Trust.

The story of Chatham Grammar School will serve as a stark warning to other schools who appear to be doing well by traditional headline measures. Coasting, it seems, may longer be an option.