Half of all school sixth forms in England will have to axe courses or face the risk of closure as a result of a major overhaul of post-16 funding to be introduced in September, headteachers have warned.
Changes to the system are expected to result in school and college budgets being cut by 15 per cent over the next three years, according to the Association of School and College Leaders (ASCL).
Heads have calculated that, as a result, it will be "unviable" to maintain sixth forms with fewer than 200 students. At present, more than 1,000 state schools - 50 per cent of those with sixth forms - fall inside that category.
Schools and colleges could be forced to increase class sizes and cut the number of A-level courses and vocational qualifications offered in order to survive, ASCL deputy general secretary Malcolm Trobe told TES.
The union fears student choice could be restricted, with some being forced to travel long distances to study specific courses.
"There is a great deal of concern among principals," Mr Trobe said. "There is going to be a significant funding reduction, which they fear will impact the courses they can offer, as well as exam results.
"It is highly likely that we will see some small sixth forms forced to close over the next few years because of lack of funding. Some of the most vulnerable will be in rural, sparsely populated areas where there are already limited options for studying post-16."
From September, schools and colleges will no longer be funded per qualification. They will receive the same basic funding for each 16-19 student, irrespective of what they are studying. Schools will no longer receive extra cash for running more expensive courses.
In a survey of more than 500 ASCL members working in school sixth forms and colleges, 25 per cent said provision would be jeopardised, with a further 68 per cent saying that the changes would "make life more difficult".
Two-thirds of respondents (67 per cent) said there would be a reduction in the number of courses they offered, with 13 per cent saying that they expected to make "significant" cuts to provision. Almost nine in 10 said they would have to increase class sizes, with 51 per cent saying this would have an adverse effect on standards of education.
The ASCL's concerns follow a warning from leading scientists - including the Royal Society, the Institute of Physics, the Royal Society of Chemistry and the Society of Biology - that the changes could dramatically reduce the number of schools offering science A levels because they will no longer be compensated for their extra cost.
As part of the expansion of the academies programme, many schools have been encouraged to open new sixth forms.
David Igoe, chief executive of the Sixth Form Colleges' Association, told TES that many schools with small sixth forms would only be able to provide a "severely restricted offer".
Of the sixth-form college leaders who responded to the ASCL survey, 39 per cent said the changes would have some negative impact on the experience of post-16 students, while 51 per cent said there would be a significant negative impact.
A Department for Education spokeswoman said the current formula had "acted as a perverse incentive for schools to enter students for easier qualifications".
"Funding schools and colleges per student instead will free them up to deliver demanding and innovative courses that meet the individual needs of all young people," she added.
Do you believe that the implementation of the new funding will affect the viability of post-16 provision?
Secure post-16 provision: 0.6%
Make life easier - 1.6%
No impact: 4.9%
Make life more difficult - 68.2%
Jeopardise post-16 provision: 24.9%
How will the new funding affect the number of courses on offer to students?
Opportunity for significant increase: 0%
Opportunity to increase: 2.7%
No impact: 17.7%
Some reduction necessary: 66.6%
Significant reduction needed: 13%.