Compulsory education or training for 16-18s will not boost most results and will only benefit the UK economy by pound;60m a year.
Five out of six teenagers forced to stay in education under proposed changes in the law in England will fail to either improve their qualifications or reach GCSE-standard, according to Government estimates.
The move will apply only south of the border. Labour in Scotland had also pledged to raise the education leaving age to 18 in its election manifesto last year. But the SNP Government has set its face against it, believing that compulsion will do more harm than good.
An assessment of the impact of raising the age to 18 found that the 83,000 students forced to stay on in England were 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: "Young people who need a little more time to gain their level two or three qualifications will be able to attend courses free after the age of 18. This and other changes will ensure there are suitable progression routes available to give all the skills they need to succeed - whatever level they are working at.
"Importantly, we know that education is associated with a wide range of non-attainment benefits, which were not estimated by this research but must be taken into account when assessing the benefits of compulsory participation. Research suggests that people who participate between the ages of 16 and 18 are less likely to experience 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 2015-16 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 spokesman said the pound;2.4bn benefits considerably exceeded the cost, estimated at pound;800m per cohort. "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 have criticised the education and skills bill plans to force young people in England 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, we are in favour of young people retaining their right to choose."