It said teaching in one in five pupil referral units (PRUs) was unsatisfactory. Management was inadequate in a quarter.

In a report, Ofsted called for a national strategy to improve the school experience for struggling, disaffected teenagers - whom it described as "some of our most vulnerable young people".

It said many schools were using the flexibility in the curriculum to try to stop teenagers dropping out - the best putting a "strong emphasis" on work-related study.

But it said there was a lack of suitable courses and qualifications for these youngsters.

And there was particular concern about "alternative education projects" - usually run by the voluntary sector for boys who, typically, have been out of formal education for more than two years, have poor skills and have often been involved in crime.

Inadequate curriculum

Ofsted said many projects were not organised well enough.

"The approach used sometimes gave the very strong impression of a concern merely to 'keep them occupied', so that activities without any educational or even social objectives, such as trips to ice rinks and bowling alleys, became a substitute for a worthwhile programme."

Providers of alternative education are dealing with vulnerable young people who very often have poor attitudes Ofsted report

And it said no pupil had access to the full national curriculum, "or anything like it".

Almost a third of the teaching was unsatisfactory, while good leadership and management was "rare".

"It must be recognised that the providers of alternative education are dealing with vulnerable young people who very often have poor attitudes," said the report.

"Alternative education projects run by voluntary bodies often have a shaky funding base and not much access to professional advice and support on the curriculum, assessment and teaching."

It said the weaknesses reflected national problems, with no mechanisms for the registration of alternative providers and no recommendations covering their work.

'Pressing problem'

Ofsted's director of secondary education, Mike Raleigh, said: "Helping youngsters like these is a tough job.

"If you were looking for a contest of the hardest bit of teaching, dealing with recalcitrant, low-achieving youngsters with poor attitudes and not much ambition would come near the top of the list."

The chief inspector, David Bell, said: "Improving the provision for low-attaining and disaffected 14 to 16-year-olds is one of education's most pressing problems.

"Many of the schools visited were taking steps to deal with the problem but there is more to do.

"It is a major concern that many providers in other settings charged with educating some of our most vulnerable young people are neither sufficiently well regulated nor adequately monitored.

"Their pupils are the most likely to be subject to a poor curriculum and teaching."

'Missing thousands'

He said "an even greater worry" was that some 10,000 "missing" children aged 15 appeared not to be receiving any education at all.

"These young people are not getting the qualifications, let alone the social and life skills that they need to progress."

In the past, such figures have been seized on by advocates of home education as evidence that many youngsters are learning with their parents. Ofsted makes no mention of this.

In response to Ofsted's report, the Department for Education and Skills said it was about to embark on a reform of education welfare which it said would have so-called 'missing children' as a major priority.

The Education Minister, Ivan Lewis, said: "Before 1997, excluded children literally roamed the streets, getting into trouble and committing criminal acts.

"We had a massive job to do to put in place a provision, which we have now done."

The government agreed with Ofsted that the next challenge was to ensure that the quality of that provision matched the best.

The shadow education secretary, Damian Green, said: "It is a sad fact that the children who are most likely to gain nothing from our schools are the poorest and most vulnerable.

"This report shows that the many initiatives of this government are not working. What we need instead is a serious commitment to vocational education at a younger age than it is now offered, and the ability to diagnose potential problems in individual children earlier."