Vulnerable pupils are being sent away from school to take unregulated courses in unsuitable settings, which are often not inspected by the teachers or local authorities who send them there, according to a new report from Ofsted.
Pupils at risk of exclusion are not monitored closely enough by school staff after being enrolled in alternative provision - lessons and courses often run by private companies or special schools - according to the study.
Inspectors carrying out the research found many pupils never meet with a teacher, despite a recommendation that they should check on students' progress.
Inspectors found one group of children studying in "cramped, cold and generally unsuitable accommodation", and another group attending a part-time placement in a building that was "poorly maintained and dark, with broken furniture and stacks of old equipment".
Most providers of alternative provision do not have to register with any official body, and there are no "consistent arrangements to evaluate their quality", Ofsted found.
The report calls for more monitoring and regulation of alternative provision.
The Department for Education has already announced that in future all providers will have to earn a "quality mark" in order to operate.
Inspectors visited 61 providers as part of their research and just 17 were subject to an inspection regime.
At 11 providers, teachers had not visited pupils at all. In 2008, the former Labour government estimated that 135,000 pupils accessed alternative provision at some point in the school year.
Ofsted found the "weakness" in monitoring and evaluation meant teachers did not notice when a course wasn't working or had been unsuitable from the start.
"Some of the schools and pupil referral units visited saw alternative provision as very separate from their own work and as a 'last resort' for a challenging student," the report said.
New Woodlands in Bromley is a special school that also runs alternative provision for children in the area. Headteacher Duncan Harper said daily monitoring was the key to helping pupils back into mainstream education.
"Alternative provision providers also need to be badgering schools about their pupils. Some schools are happy to pay providers just to keep children there. I agree with Ofsted that children in alternative provision can become out of sight, out of mind," he said.
Malcolm Trobe, policy director of the Association of School and College Leaders, said a new database and a quality mark for providers would be a "very positive step".
"It's difficult for individual schools to make a decision when it's their first experience of using alternative provision. They find it very time-consuming to look at a range of providers," he said.
A DfE spokesman said: "As set out in our white paper, we want to increase the autonomy, accountability and diversity of alternative provision to help drive up standards. We will study Ofsted's recommendations in detail over the coming weeks and make our formal response by early autumn."
Ofsted visited 39 schools and pupil referral units as part of its research into alternative provision.
Almost 70 per cent of pupils in Years 9 to 11 attending courses away from school had special educational needs. A third had already received a fixed-term exclusion.
In 14 of the 39 schools, more than 75 per cent of pupils in alternative provision had made at least satisfactory progress in literacy, numeracy and ICT.
This figure was roughly 50 per cent in 12 other schools.