It is more than two decades since the school-leaving age was raised from 15 to 16, but for many British teenagers such formalities mean nothing. This autumn, like every autumn, thousands of teenagers decided that enough was enough and, despite the rules, quit.
While their former colleagues sit through key stage 4, willingly or otherwise, they are hanging around shopping centres, earning a pittance on Christmas market stalls - if they are lucky - or building a new life of petty crime. Sometimes called "the black hole kids", these are education's disappeared and most will stay that way.
Speaking in 2001, the then chief inspector Mike Tomlinson put their numbers at 10,000, expressing serious concern. Last summer his successor, David Bell, repeated the point, using the same figure. Last month the crime reduction charity Nacro came out with a study suggesting that between 50,000 and 100,000 pupils of all ages are missing from schools in England and Wales.
This report, Missing Out, coincided with the closing date for the Government's Green Paper on children's services. This is a document which puts the phenomenon of these missing pupils in an even more serious light because it comes in response to the violent abuse and murder of eight-year-old Victoria Climbie. It seems that Victoria was never even sent to school.
The paper, which will lead to a children's Bill in the new year, demands that those responsible for health, education, justice and social welfare co-ordinate their activities, and calls for a comprehensive tracking system of pupils as they move around from area to area.
It is a big task. The problem is, by its nature, hidden - and deliberately so. These are not pupils who have been formally excluded, jailed or hospitalised because such children are, if nothing else, on a list somewhere. Instead, the chief inspectors were talking about teenagers who are self-excluding, who want to vanish and do so very successfully.
The teaching unions and headteachers' organisations, often a good source of research, frankly admit they know little about the problem. But individual heads like Anne Welsh, head of George Stephenson high school in Newcastle-upon-Tyne and president of the Secondary Heads Association, confirms that it is an annual concern. Every year, she says, three or four of her older pupils stop turning up and, despite the best efforts of the school and local authority, they never come back.
John Stead, education co-ordinator for the NSPCC and himself a former comprehensive school head from the North-east, had a similar experience.
Camila Batmanghelidjh, the founder of Kids Company charity in south London, has found "an underworld of children who are coping with an enormous amount, children who know about each other. But adults don't necessarily know about these children and their lives. There's a gulf between the information on files and what children know about each other."
In the case of younger pupils, there might well be parents who have purposely removed them from the gaze of prying eyes - a particular concern of the child-protection charities such as the NSPCC and Kidscape. They say it is alarmingly common for families under suspicion to vanish overnight.
The scale of the problem is uncertain. The Office for Standards in Education figure of 10,000 is a statistical projection from school census returns, produced by the Department for Education and Skills in 2001. A DfES spokeswoman says this and other figures should be treated with caution.
The Nacro figure of 50-100,000 is similarly shaky, based on the 380 children that Blackpool local education authority reckoned were missing in one year. But there is little doubt about what kind of child is most at risk because they are the ones most vulnerable in every aspect of life.
The leading charities in the field, Turning Point, Kids Company, the NSPCC, NCH and Nacro, for example, say that they find a multiplicity of problems.
A series of overlapping themes emerges, including bullying, drug abuse, family breakdown, violence, sexual abuse, low educational attainment and poor social skills.
Boys are overwhelmingly the most likely to go missing, although the number of girls is fast rising. Children from local authority care or Caribbean backgrounds appear to be at particular risk. Poverty is another factor.
Turning Point, the UK's largest social care charity, deals with 20,000 young people a year.
It recently reported that nearly 4 million children are living in homes below the poverty threshold; that up to 300,000 children in England and Wales have one or both parents with serious drug problems; and that one in four young people aged 11 to 15 has used drugs at least once.
Schools and the way the education system is structured have taken much of the blame. Nacro is one of many voices calling for a more flexible and vocationally-oriented national curriculum to reduce the drop-out rate.
Mr Tomlinson, who first brought the issues to public notice, will have a contribution to make when he publishes his review of 14-19 qualifications next year. Nacro also suspects that it can be in the interests of the school to "lose" those children who present difficulties, particularly with the added pressure of league tables. It does not help that the pupil retention grant, specifically aimed at persuading schools to hang on to challenging pupils, has now been abolished.
For its part, the DfES points to "a whole range" of measures taken to address the problem, including behaviour and learning support units, learning mentors and a drive to eliminate truancy.
Ms Batmanghelidjh believes the criticism of schools is unfair. In her view the scale of the neglect facing children and the damage it causes them has not yet been recognised.
In one south London comprehensive she worked with recently, there were 110 children on the borough's at-risk register. Every year Kids Co deals with 3,000-4,000 children coping with serious trauma as part of the charity's schools consultancy work. At the same time, its after-hours centre for children who have opted out of school altogether - and often home as well - takes 500 a year.
A near breakdown of the statementing system at primary level is another factor, she says. The sheer weight of claims for extra help with behavioural problems and dyslexia means that children's needs have not been addressed. The NSPCC, too, believes that early identification is essential, and too often missing.
Rather than organising truancy sweeps, Ms Batmanghelidjh says the Government should pay heed to the evidence emerging from new brain-scan techniques and other scientific developments about the lasting neurological damage caused by systematic neglect and abuse. Once the damage is done, the task of repairing it is beyond what most secondary schools can manage.
"They can't. It's not fair to put the burden of care on the schools, partly because of the way they cost these children," she says. "The truth is that children like that can cost pound;8,000 a year to deal with. Schools simply don't have the resources.
"These are not 'OK' children who make a choice to disappear. Schools are not in a position to meet their needs."