The failure of many young people to "engage in education" is a major problem, a report on teenagers' learning in England and Wales says.
The report argues for greater recognition of vocational learning
The latest Nuffield Foundation review says the failure is due partly to "inappropriate targets, learning experiences and forms of assessment".
But it praises the "learning pathways" in Wales, and England's recognition of different kinds of achievement.
It reiterated concerns that there is too much "policy busyness".
English Schools Minister Jim Knight said on Thursday the way boys' attainment lagged girls' might be down to a lack of motivation in adolescence.
There was a tendency for government departments and agencies to address symptoms of deep-seated problems rather than underlying causes, the new report said.
The "unprecedented" amount of policy initiatives such as national targets, new qualifications, short-term funding and new regulations was "unlikely to produce significant improvements to the education and training system as a whole".
"A full assessment of the impact, cost, and the problems associated with introducing such a large number of initiatives should be carried out," it said.
Although partnerships between schools and colleges had improved vocational learning opportunities, there was still too much competition rather than collaboration between institutions.
"The system in England could still be described as weakly collaborative with weak governance at the local level," it said.
It advocated a wider range of partners including youth service, voluntary bodies and independent training providers, and the development of "strongly collaborative local learning systems".
The 14 to 19 reforms in Wales covered both general and vocational education, and the system was more locally determined rather than centrally driven - but the two countries were still tied through shared qualifications and university systems.
The report calls for clearer routes into higher education for those with advanced vocational qualifications.
A spokesman for the Department for Education and Skills said assessing pupil progress was necessary to achieve higher standards, as seen in this year's GCSE results.
"We need to make sure that the system is working for every teenager and that is why we are taking forward radical changes to 14-19 learning that will further raise attainment, tackle 'drop-out', ensure that every child leaves school with the basics and provide more choice for young people."
The general secretary of the National Union of Teachers, Steve Sinnott, said there were still "knotty issues" in 14 to 19 education which needed tackling.
"Despite the introduction of specialist diplomas in 2008, there are still likely to be 16-year-olds not in education, employment or training whose needs will remain unmet."
He said the government should "show some courage" and reflect on what could still be salvaged from the Tomlinson Report, which advocated diplomas that would replace existing qualifications.
Liberal Democrat education spokeswoman Sarah Teather said league tables had created perverse incentives, with schools forced to focus on their ranking rather than doing what was best for their pupils.
"Collaboration between schools, colleges and workplace learning providers and an overarching diploma to replace outdated GCSEs and A-levels would mean more options for young people," she said.
The independent review is being extended for a further three years.
Nuffield Review of 14-19 Education and Training, Annual Report 2005-06.