Students with exams this summer should study for seven hours a day over the Easter holidays, an education expert has said.
Barnaby Lenon, chairman of the Independent Schools Council, published the advice on the council's website.
He said: "That may seem a lot, but the students who get the best grades will be working this hard."
But teachers and other experts said it was "unrealistic" and could put too much pressure on students.
Mr Lenon, who retired as headmaster of Harrow School in 2011, offered five tips which included starting at 9am and working for no more than two hours at a time before taking a break.
He said: "The best GCSE and A-level results don't go to the cleverest students - they go to those who revised in the Easter holidays."
His revision schedule would mean 100 hours of work over 14 days in the Easter holiday, with repeat sessions in the summer term and before the exam.
Stephen Cumper, a teacher from Belfast, said the advice was "complete nonsense".
"Children need to have a work-life balance," he said.
Colleen Shelley, a teacher in a private girls school in Hertfordshire, said: "That's absolutely not acceptable. It's not realistic.
"We as adults don't function like that in a work environment. Work should be intermittent - revise in moderation and take breaks."
Sarah-Jayne Blakemore, a psychology professor at University College London who researches the adolescent brain, said teenagers were already under a lot of stress.
She said on Twitter: "This generation of young people are under massive pressure, academically, socially... I'm not sure recommendations like this help much."
One parent, Serian Walker, who is also a police officer, said in her profession she had dealt with suicides by children who could not cope with the pressure.
Ms Walker, from Cardiff, said she struggled to get her 15-year-old daughter to take a break from revision.
"I know she isn't sleeping well and I know she's miserable. Of course I'd like her to excel, but not at the cost of her mental health," she said.
But Clare Wagner, head teacher of the West London Free School, endorsed Mr Lenon's tips as "excellent revision advice".
One recent A-level student also backed Mr Lenon's advice.
Natasha, now in her final year at Queen Mary University of London, said that she relied on at least seven hours of study a day in the Easter holidays during her GCSEs and A-levels.
"Looking back, there is no way I would have been able to properly cover all the content you need to revise in fewer than seven hours a day. I can't see how you can get top grades and study fewer hours," she said.
"It is not about finding a work-life balance, it is about making a sacrifice."