Schools in England are focusing on tests and exams, rather than giving pupils a good grounding in a wide range of subjects, the head of Ofsted warns.
Amanda Spielman says the focus on GCSEs and national curriculum tests, often known as Sats, is at the expense of "rich and full knowledge".
She accepts a good school curriculum should lead to good exam results.
But she says good exam results do not always mean children have received the subject knowledge they need.
Her comments came as she set out the preliminary findings of an Ofsted research project into the curriculum in England's state schools.
Is the Ofsted boss saying testing is bad?
No. She says: "Testing in school clearly has value."
However, she is encouraging schools in England to focus less on drilling pupils through past papers and more on widening their knowledge and horizons.
"The regular taking of test papers does little to increase a child's ability to comprehend," she says.
"A much better use of time is to teach and help children to read and read more."
Why has she come out and said this?
Ms Spielman says schools have a duty to develop each individual child and give them a broad education.
"A good curriculum should lead to good results," she says.
"However, good examination results in and of themselves don't always mean that the pupil received rich and full knowledge from the curriculum.
"In the worst cases, teaching to the test, rather than teaching the full curriculum, leaves a pupil with a hollowed-out and flimsy understanding."
She also says a rounded education - or the lack of one - has consequences for social mobility, with less academic children being particularly hard-hit if schools drop subjects such as art to focus on core ones.
"Restricted subject choice for low-attaining pupils disproportionately affects pupils from low-income backgrounds," she says.
Will anything change?
It certainly looks likely - Ms Spielman is in a high-profile job and her viewpoint carries weight.
While she makes clear the school curriculum is the responsibility of the Department for Education, she indicates schools will see a shift in the emphasis and tone of their next visit from her inspectors.
"School leaders need to recognise how easy it is to focus on the performance of the school and lose sight of the pupil," she says.
"I acknowledge that inspection may well have helped to tip this balance in the past."
What is the history of all this?
Three years ago, a new curriculum was introduced across schools in England. It was the brainchild of former Education Secretary Michael Gove. The aim was to raise standards.
In primary schools, the end-of-school Sats, became harder, with more focus on the "three Rs" - reading, writing and maths.
The first raft of "tough" tests were taken in 2016, and just over half (53%) of 11-year-olds made the grade.
In secondary schools, the amount of coursework was reduced and the focus shifted to an end-of-course exam rather than taking tests along the way.
The first round of new GCSEs - in English and maths to start with - were taken this summer, with the pass rate dropping in both subjects.
What is the response from teachers to the Ofsted chief's comments?
Teaching unions have long argued that testing - with results directly linked to school league tables - means teachers end up teaching to the test.
Responding to Ms Spielman's comments, Geoff Barton, general secretary of the Association of School and College Leaders, said: "It's hardly surprising that schools focus intensely on Key Stage 2 tests and GCSEs as that's how their performance is measured, with GCSEs crucial to the life-chances of their pupils.
"If Ofsted wants them to focus less on these assessments, we would suggest it lobbies the government for a change to the accountability system rather than criticising schools.
"Everybody wants to teach a broad curriculum. It is essential that we have an accountability system which supports rather than narrows that aspiration."
Nick Brook, deputy general secretary of the school leaders' union NAHT, added: "We must value a broad range of subjects in the school day so that pupils' opportunities are not limited.
"School leaders want to do what is best for the children in their school but are restricted by an accountability system which places undue weight on SATs results."
What is the government's reaction?
The DfE says all schools should provide a broad and balanced curriculum for all children and that the best schools are doing that.
"Our reforms are ensuring children are taught the knowledge and skills they need to fulfil their potential, and Ofsted's chief inspector recognises the importance of these reforms.
"We are beginning to see higher standards right across the school system with more six-year-olds reading fluently; more primary school pupils achieving the expected standard of a rigorous primary curriculum; and secondary school pupils rising to the challenge of more demanding GCSEs in maths and English."