Major reforms to GCSE exams risk losing the "real substance of education" as schools take a results-driven approach to teaching, Ofsted's head has warned.
Chief inspector of schools Amanda Spielman expressed concern that the shake-up of grading, introduced this summer, could deprive pupils of a broad education.
Traditional A*-G grades have been axed and from this year students saw GCSEs in English and maths graded 9-1 - with 9 the highest result. In the next few years, the changes will be brought in for all subjects.
Supporters have argued the move is necessary to allow more differentiation between students.
Extending the grading system is expected to make it tougher for pupils to crack into the highest bands.
Ms Spielman told The Sunday Times pressure to succeed under the new regime meant schools were finding it "hard to make sure they put children's interests first and think children, children, children".
"The real substance of education is getting lost in our schools," she added.
An inclination to drill students for exam success could follow, she told the paper, compromising their chance of getting a "broad and balanced education".
She claimed to have seen a class off 11-year-olds being led through the GCSE mark schemes in place of their normal lesson at one school.
Schools were also said to be broadening some courses from two to three years so pupils could be adequately prepared.
This method might ensure they get "cracking grades", she said, but it could leave them short of the skills required to thrive in later life.
Earlier this year, teachers' union NASUWT said at its annual conference that the reforms could do more harm than good.
General secretary Chris Keates said at the time: "The Government has consistently sought to portray GCSEs as 'broken' and 'dumbed-down' qualifications in order to push through its vision of an elitist, narrowly focused curriculum and qualifications system which risks failing to meet the needs of the majority of young people.
"Schools already buckling from excessive workload are now facing even more bureaucratic reform and young people, already experiencing rising rates of anxiety and mental ill health, will face even greater pressure to perform."
A Department for Education spokesman said: "This summer, pupils have taken new GCSEs in English and maths which are benchmarked against the best in the world.
"This ensures pupils are taught the knowledge and skills to succeed in work and further study in 21st century Britain.
"These exams are the culmination of four years of reform and collaboration with the sector, starting in 2011 with the national curriculum review in England, and involving extensive consultation with schools, HE and employers."