Teenagers are to be forced to stay on at school until they have passed their English and maths GCSEs, in a move which has raised "serious concerns".
Under the new reforms, published by the government on Monday, the school leaving age will be raised to 17, and will increase to 18 in 2015. One A-level student told the Huffington Post UK he believed the reforms would be "detrimental to all".
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16-year-olds who do not get a C grade or better in their English and maths GCSEs will be told that they must sit the exams until they gain the key qualifications.
The government said that the move will help ensure that young people have a good grasp of English and maths, but the general secretary of the Association of School and College Leaders (ASCL) said there were "unanswered questions" remained.
"We have been expressing serious concerns to the Government for some time about the implementation of this very significant new policy about which there are many unanswered questions in the absence of a coherent and funded implementation plan," Brian Lightman said.
"At a time when post 16 funding is being significantly reduced and feedback from ASCL members continues to show very different states of readiness in different parts of the country it is difficult to see how schools, colleges, employers and local authorities will be able to provide additional classes or recruit suitably qualified teachers."
The proposal was first put forward by Alison Wolf, the Sir Roy Griffiths Professor of Public Sector Management at King's College London, in her 2011 review of vocational education.
At the time, she said it was "scandalous" that half of 16-year-olds were leaving school without good GCSEs in English and maths, and warned that it was a real failure of the education system that many of these youngsters would still not have these qualifications at age 18.
Prof Wolf's report called for any teenager who did not get C grades in GCSE English and maths to continue with them post-16 as the two subjects are the "most important in the world".
This recommendation was the "single most important", Prof Wolf said on Monday, adding that she was "delighted" to see it being implemented.
Figures show among young people aged 19 last year, 285,000 had left school at age 16 without a C or higher in both English and maths, and by the time they were 19, 255,000 had still not reached this level, the Department for Education (DfE) said.
Around a fifth (21%) of those who had not gained a good grade at age 16 in English continued studying it, along with 23% of those who had not got a C or better in maths.
The reform will be introduced from the start of this term - which begins this week for many schools - and comes as the education participation age is raised to 17. In 2015 it will be raised to 18.
Christine Blower, General Secretary of the National Union of Teachers, said: "While it is of course desirable that young people gain GCSE passes in English and Mathematics, there will be some, particularly those with special educational needs, for whom achieving a C grade will simply not be possible.
“It is critical that the Government works with schools and colleges, to make sure that what is on offer for those young people is not just marching them in and out of the exam room but actually making sure they are developing by having an appropriate curriculum.”
The DfE said that ideally, teenagers without C grades or higher in English and maths will continue studying for GCSEs in these subjects, although they can also take other qualifications such as functional skills and maths courses accredited by the exams regulator Ofqual as a "stepping stone" to GCSEs.
The English and maths results of 16-19-year-olds who did not gain these key GCSEs will also be reported in annual school league tables, the department said.
Education Secretary Michael Gove said: "Good qualifications in English and maths are what employers demand before all others. They are, quite simply, the most important vocational skills a young person can have. Young people must be able to demonstrate their understanding of these subjects."
Prof Wolf said that the move will have a "hugely positive impact on the ability of hundreds of thousands of young people to get good jobs."
The change comes amid growing calls from a number of groups for all youngsters to continue studying maths after GCSE.
Mr Gove has previously said that within a decade, he wants to see the vast majority of teenagers studying maths up to the age of 18, and the Government is developing a new set of post-16 qualifications in the subject.
A poll published by the Sutton Trust last month found that 64% of 11-16-year-olds in England and Wales were in favour of young people at school or college studying maths and English up to age 18.
At the moment, around one in five young people in England continue studying maths past the age of 16, compared to other developed nations where the majority of students continue the subject.
However, Prof Wolf said: "Institutions no longer have an incentive to steer students away from anything that might be a bit difficult or that they might possibly fail - like GCSE maths or English.
"If qualifications of limited value get dropped and replaced by GCSE maths and English in a student's overall programme , that's all to the good.
"They should be top priority in any student's programme, and delaying the implementation of this reform would be a betrayal of another generation of students."