Up to 10,000 children of school age are missing out on full-time education and in danger of becoming "invisible" to the authorities, according to a report by inspectors published today.
The study, from education standards watchdog Ofsted, warned they could more easily become prey to abuse as local authorities failed to keep a track of them.
"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," said chief schools inspector Sir Michael Wilshaw in a foreword to the report - Pupils Missing Out On Education.
It found that four out of 15 authorities surveyed did not have a clue as to how much education provision children out of school were receiving. Only five had their finger on the pulse and could tell inspectors immediately the state of provision.
Some of the children concerned had been excluded from school illegally, the report added. The children were sent home from school without heads operating the formal legal exclusion process.
Others included pregnant schoolgirls and children with mental health problems, it added. In one case, a 15-year-old boy had not been receiving full-time education for eight years. Ofsted described the situation as "clearly unacceptable".
In the 15 authorities, a total of 1,400 young people were failing to receive a full-time education - getting between five and eight hours of learning a week. If this was an accurate reflection of the national picture, it would 10,000 children were in this position.
The report recommends putting a full-time official in charge of delivering full-time education for those children not in school.
In addition, it says that - in future - Ofsted will inspect out-of-school provision for vulnerable children itself.
"The best local authorities ensure that no young person in their area slips out of sight," the report adds.
"They are conscientious and determined in communicating with others, understanding that such responsibility does not stop at their local authority's boundaries."
However, it added: "In a number of cases, low expectations of what children and young people could achieve meant the education provided was of poor standard and limited value.
"For example, one young mother had stopped studying for her exams where she had been on track to do very well and, instead, was attending part-time provision for parenting skills."
"Many thousands of children and young people in England do not attend full-time education," concluded Sir Michael. "Too often, children and young people who receive only a part-timer 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."
Sarah Lambert, head of policy at the national autistic Society, said: "The report's findings are stark and sobering."
Many parents of autistic children felt they had no option but to educate their children at home because no suitable school provision was available, she added.
She added: "Sadly, when parents take their child out of the school system they often find that their local authority refuses to provide any further educational support to educate their child."