Some may be in colleges, working or on the streets
As many as 25,000 teenagers disappear from school rolls in England when they are 14, a leading educationist says.
Sir Mike Tomlinson, a former chief inspector of schools and head of Hackney Learning Trust, says they feel school "has nothing to offer them".
The law says they should be in full-time education to age 16, and that is being changed to require some form of learning to age 18.
A charity has called for more courses for youngsters who have limited skills.
Sir Mike made the remarks at the Chartered London Teacher Conference.
He explained that as pupils were tracked from year to year in the annual census that all schools are required to complete, the total numbers fluctuated only slightly up to Year 8 or 9, when they are aged about 12-13.
But then there was a drop.
He said: "There are around 25,000 who fall off the rolls when they move from Year 9 to Year 10.
"They are saying 'this is no place for me'.
"They end up in poorly paid jobs or with no jobs at all."
He added later: "In general terms we don't know where they are.
"They may be in college and have persuaded a further education college to take them on, they may be working in their parents' business, or they may be on the streets."
He said schools did attempt to find them but this obviously was not successful.
"They are very, very worrying figures," he said.
Liberal Democrat schools spokesman David Laws said: "Thousands of young people are dropping out of school early because the education system has let them down.
"This shows just how ludicrous it is for the government to raise the education leaving age when it can't even get 14 year olds to turn up."
Seven years ago, when Sir Mike was head of the Office for Standards in Education (Ofsted), he suggested some 10,000 pupils were missing from the system.
He said he believed then that the problem was growing.
Last year a committee of MPs - investigating the issue of forced marriages - was told that more than 2,000 children were estimated to be missing from schools in about a tenth of England's local authorities.
The charity UK Youth, which offers skills training to some 750,000 young people, has said things may get worse if more is not done to recognise those at the lower end of the attainment scale.
It said the Apprenticeship Bill, currently being given its second reading in the House of Commons, put too much emphasis on qualifications.
Chief executive John Bateman said: "These young people need more stepping stones to validate and recognise their achievements before they can aspire to reach NVQ Level 2.
"Building the necessary skills and confidence to carry out a simple task such as baking a cake is often viewed as a minor learning goal, but this can be accredited and provide the confidence and self-esteem necessary to progress to formal qualifications."
This is the approach being taken in Scotland where the government is proposing to vary education maintenance allowances to encompass those in community and voluntary projects.