Pupils at a Merseyside private school went into an English Literature GCSE exam having been taught the wrong book.
The school spotted the error once the exam began
Birkenhead School, which charges £7,647 a year, has begun an inquiry into its English department.
Fifth-formers at the boys' school had been expecting to answer questions on John Steinbeck's Of Mice and Men.
The exam board, OCR, said the text had been on the approved list but had been replaced in January and that the school had been notified twice.
The students should have been taught one of Things Fall Apart, by Chinua Achebe, The Old Man and Sea, by Ernest Hemingway, or George Orwell's 1984.
Twenty-four pupils at the Oxton school were affected by the error, although the school said of the two questions listed, the students could answer both.
"We have to hold our hands up here and say it's our fault and we will do all we can," said deputy headmaster, David Edmunds.
"We contacted the parents of the affected students ourselves to explain the problem. We have begun an internal investigation to ensure this does not happen again.
"The exam boards are used to this happening. It is not unusual to find pupils studied the wrong text."
Mr Edmunds added that no members of staff had been suspended and he "hoped" no-one would lose their job.
'Marks not affected'
Bozena Hillyer, vice president of the school's parents association, said: "Given how good the school is it's surprising but people do make mistakes."
A spokeswoman for Oxford, Cambridge and RSA (OCR) said pupils' marks were not affected by their teachers' mistake.
"It's one of those things that's not really our fault in this case," she added.
"The school has made an error on their part. We've done all we can to communicate with the school, they've had at least two notices this year."