Thousands of pupils could be "disappearing" from the school system as a result of illegal off-rolling, Ofsted has warned.
Inspectors found that 19,000 children dropped off school rolls between January 2016 and January 2017, during the time students take their GCSE exams.
Around half (9,700) of those dropping off rolls between Years 10 and 11 are not reappearing on the roll of another state-funded school.
Off-rolling, where schools move difficult-to-teach pupils off their rolls to boost performance data, is illegal.
The findings in Ofsted's annual report, to be published later today, are of particular concern in London where there is a higher proportion of schools with pupils moving than in the rest of the country.
Ofsted has identified 300 out of 2,900 schools that lost pupils between Years 10 and 11 as having particularly high levels of movement.
A spokeswoman for Ofsted said the practice occurred when school leaders "lost sight of their core purpose and put the school's interest ahead of the child's".
Ofsted said a pupil could be removed from a school roll for various reasons, such as families moving home or parents deciding to home-educate them.
It recognised that many pupils might move to independent schools or home education, but others could end up in an unregistered school or drop out of education entirely.
The Ofsted spokeswoman said it seemed "unlikely" parents would voluntarily choose to home-educate their children or send them to another school in the middle of their GCSE courses.
"Which is why the magnitude of these numbers is such a cause for concern," she added.
Ofsted has found that vulnerable children, those with special educational needs and disabilities (SEND), those eligible for free school meals and some minority ethnic groups, are more likely to leave school.
Around 30 per cent of the 19,000 pupils leaving school rolls have SEND, compared with 13 per cent of all pupils, and 54 per cent are eligible for school meals, compared with 28 per cent of all pupils.
Ofsted said it would use the its data to prioritise which schools to inspect, adding that its new education inspection framework for September 2019 will make it easier to reward schools for good work and shift the focus away from performance measures in isolation.
Inspectors also have concerns about the "worrying" gaps in SEND provision across the country.
There are 1.3 million pupils in England with SEND, about 15 per cent of all pupils, with roughly 1 million receiving SEN support, and 250,000 having an education, health and care (EHC) plan.
Pupils with SEN are five times more likely to be permanently excluded than other pupils overall, Ofsted found.
In 2017, more than 4,000 children with an approved EHC plan had received no provision, five times more than in 2010.
Many local areas have not implemented the 2014 SEND reforms, with the quality of EHC plans varying across the country.
The Ofsted spokeswoman said the statistics presented a "bleak picture" and it was a "scandal" so many pupils were waiting for support.
"We have also seen some EHC plans that are wholly inadequate and an insult to children and their families," she added.
Chief inspector Amanda Spielman, who will present Ofsted's annual report today, is also due to say that schools cannot be a "panacea" to all "societal ills".
Health professionals, parents and safeguarding partners should all play a role in protecting, educating and preparing children for adult life, she will say.
Ms Spielman believes that expectations on schools to address obesity, child neglect and gang-related violence risks distracting them from their core purpose and results in a failure to solve such problems.
"Our education and care services don't exist in isolation from the local areas they serve," Ms Spielman is expected to say.
It will be her second annual report since replacing Sir Michael Wilshaw as Ofsted’s chief inspector.
Heads have called on Ofsted to start looking for links between funding cuts and education standards.