A minority of schools are still gaming the system and putting their own interests above that of students, Ofsted has warned.
The watchdog has raised concerns about schools putting grades at the expense of a well-rounded education.
It warned that some are failing to act with integrity, with England’s most disadvantaged pupils most likely to lose out as a result.
In its latest annual report, Ofsted paints a picture of improvement in the nation’s education system – with 86% of schools rated good or outstanding, along with 96% of nurseries and childminders and 81% of further education and skills colleges.
But it said that while most teachers and school leaders do the right thing and enter pupils for the qualifications that are right for them, “a small minority do not”.
“In schools, if we see actions that appear to be taken in the school’s own interests, this may be a type of ‘gaming’, by seeking to make things look better than they are,” the inspectorate says.
The report adds: “Some actions that schools take may have a beneficial impact on the school’s performance data but be of limited benefit or even at odds with the best interests of pupils.”
Ofsted chief Amanda Spielman said: “Exam results are important but have to reflect real achievement. We should not incentivise apparent success without substance.
“This doesn’t represent a good education for any child.
“For those who aren’t being read a different story every night, who aren’t taken to the museum at the weekend, who don’t get the chemistry set for Christmas, it is especially impoverished.
“These children need and deserve a proper, substantial, broad education, for as long as schools have them.”
She said Ofsted recently inspected a school that required every child to take a sports science course, regardless of their interest in the subject, while others require almost every child to take a qualification in English for Speakers of Other Languages, even if though they were nearly all native English speakers.
Ms Spielman went on to say: “We mustn’t succumb to the seductive but wrong-headed logic that we help disadvantaged children by turning a blind eye to schools that narrow education in this way, as long as they deliver acceptable grades at the end.
“Grades are hollow if they don’t reflect a proper education underneath.”
Ofsted introduced a new inspection regime last September, which focuses more heavily on the quality of the curriculum on offer, rather than exam results.
Last week, the system faced criticism from two leading academy bosses.
The chief executives of the Harris Federation and Outwood Grange Academies Trust – which collectively educate tens of thousands of pupils – told the Times they were concerned that inspectors are putting a broad curriculum over properly preparing students for their GCSEs.
They raised particular concerns about the effect on children from disadvantaged backgrounds.
In its annual report, Ofsted also says it still has concerns about the numbers of pupils leaving schools without another to go to and the practice “off-rolling”.
This is when a pupil is removed from a school’s register without being formally excluded, or a parent is encouraged to remove a child from a school when the removal is in the interests of the school rather than the youngster.
Ofsted’s latest figures show that 20,000 children left their state schools between Year 10 (2017) and Year 11 (2018).
For around half of these pupils, it was not possible to see where they had gone because they had not moved to another state-funded school, the report says.
They may have moved to a private school or into home education, Ofsted says, but adds it cannot be sure.
The report says that there are 340 schools with “exceptional levels of pupil movement”, of which around 100 have been inspected this year.
Of the 20,000 pupils who left their school overall, 22% were in one of these 340 secondaries, it adds.
It does note that high levels of pupil movement are often for legitimate reasons, such as families leaving an area, pupils moving to a specialist school, parents waiting for a place in their first choice of school and parents choosing to home educate.
Ofsted published five inspection reports last year that refer directly to “off-rolling”.
In these cases, the report says, inspectors found that some pupils left “to primarily serve the schools’ best interests, not the pupils”.
The report also touches on unregistered schools, with three cases successfully taken to court and resulting in convictions.
But it adds that Ofsted still urgently needs stronger powers to seize documents and for the government to tighten the legal definition of a school and of full-time education.