Compulsory education or training for 16 to 18s will not boost results for most and will only benefit economy by pound;60m a year.
Five out of six teenagers forced to stay in education under proposed changes in the law will fail to either improve their qualifications or reach GCSE-standard, according to Government estimates.
An assessment of the impact of raising the age of compulsory education and training to 18 found that the 83,000 extra students forced to stay on are less likely than their peers to gain qualifications after 16.
Research by the Department for Children, Schools and Families and the University of Sheffield calculated that only 17 per cent of such students were expected to reach a first level two qualification (equivalent to five good GCSEs) or improve on previous results, based on the prior low achievement of most students who drop out at 16.
A spokesman for the department said it was possible that some might pick up qualifications below level two, but that the department believed the published estimates were "reasonable".
He said: "Young people who need a little more time to gain their level two or three qualifications will be able to attend courses free of charge after the age of 18. This and the other changes will ensure there are suitable progression routes available to give all young people the skills they need to succeed - whatever level they are working at.
"Also, importantly, we know that education is associated with a wide range of non-attainment benefits, which were not estimated by this research but are nonetheless important to take into account when assessing the benefits of compulsory participation.
"Research suggests that young people who participate between the ages of 16 and 18 are less likely to experience teenage pregnancy, behave anti- socially, be involved in crime or go to prison."
The additional qualifications would add up to pound;2.4 billion extra for the economy over the lifetime of the 201516 cohort of 16-year-olds, who will be the first to feel the full effect of the changes.
But that figure works out at an additional contribution to the economy of just pound;60 million per year - less than a hundredth of 1 per cent of GDP.
The researchers also admit that estimating the economic benefits requires making a large number of assumptions, so "there is a large degree of uncertainty attached to the central estimate of the benefits".
But the DCSF spokesman said the pound;2.4bn benefits considerably exceeded the cost, estimated at pound;800m per cohort.
"The point must be made here that although there is a sound economic case for raising the participation age, this is not the only rationale for government intervention," he said. "There are wider benefits to society of having a well-educated population."
Despite these benefits, organisations representing young people and students have criticised the education and skills bill plans to force young people to stay on in school, college or work-based training. A survey by the British Youth Council, which represents 170 national organisations for children and teenagers, found nearly half of students opposed the change.
Rocky Lorusso, spokesman for the council, said: "Research shows there is not enough support among young people for this policy and it is unlikely that young people will be motivated by plans they don't agree with.
"We are not against young people remaining in education until 18, but we are in favour of young people retaining their right to choose."
He also feared that the measure would force students to take up apprenticeships, which are not eligible for the minimum wage, instead of jobs that pay the full rate.