Part-time pupils who are overlooked by local authorities risk becoming "vulnerable to abuse", according to the chief inspector of Ofsted.
Sir Michael Wilshaw warned of "invisible" pupils who were unable to attend full-time education and urged "everyone" to take "greater responsibility for knowing where they are".
Too often, children and young people who receive only a part-time education, or who have none at all, can become invisible to the local authority.
This can be a safeguarding as well as an educational matter. If no-one in authority knows what education these children and young people receive each week, or whether they even attend, they not only miss out on education but can be vulnerable to abuse.
Everyone must take greater responsibility for knowing where they are."
Pupils who do not attend school in the traditional way are at risk of becoming "invisible", Ofsted's chief inspector has warned.
In a report into part-time pupils, Ofsted found many local authorities did not know how much education children who had been permanently excluded, were pregnant or young mothers, those with special educational, physical or mental health needs were getting.
Sir Michael Wilshaw there is "no greater responsibility than to ensure our most vulnerable children have the best chance of a decent education".
"It is simply not acceptable that only a third of local authorities have a detailed understanding of what is happening to pupils who are not receiving full-time education. Ofsted is shining a spotlight on these failings."