Scores of small school sixth forms face closure as a result of the government decision to lower the rate of funding for post-16 education, experts claim.
The warning comes as organisations representing post-16 institutions - including the Association of School and College Leaders (ASCL), the Association of Colleges (AoC) and the Independent Academies Association (IAA) - write to education secretary Nicky Morgan and chancellor George Osborne to express concern that funding levels are "fundamentally too low".
Ministers encouraged schools to create their own 16-18 provision as the compulsory participation age for education in England was raised to 18. Since 2010, 169 schools have opened sixth forms.
But although school budgets have been protected for pupils up to the age of 16, sixth-form provision was cut significantly under the coalition government. Last year, funding for 18-year-olds attending school or college was reduced: per-pupil levels are now 17.5 per cent lower than for 16- and 17-year-olds.
Low funding has already rendered provision unviable in some schools. Icknield Community College in Oxfordshire was forced last month to abandon plans to open a sixth form in September. Headteacher Mat Hunter had hoped to begin with a cohort of 30 students, eventually growing to 120, but decided to drop the project because of budget changes.
A similar picture is emerging across the country as school sixth forms struggle to meet their curriculum obligations with shrinking funding. Culcheth High School in Warrington shut its sixth form as a result of competition from other providers. Many more are expected to follow suit.
According to the ASCL, sixth forms with fewer than 200 students will be "financially unviable" under present funding levels. Across England, more than 1,200 school sixth forms currently have fewer than 100 students on roll.
Sewell Park College in Norwich is currently in the process of closing its sixth form - Jeremy Rowe, headteacher of highperforming Sir John Leman High School in nearby Suffolk, was drafted in to help. Mr Rowe said running a school sixth form was difficult. "You have to really believe in it, because if you don't there's no point and it will only be a problem for the rest of the school," he added.
In December, Ofsted hit out at the quality of provision in small school sixth forms, warning that they "frequently offer too narrow a range of subjects or teach those subjects poorly".
Malcolm Trobe, deputy general secretary of the ASCL, said that more money was needed to avoid further closures. "The likelihood is that we'll see a number of smaller sixth forms close because of financial pressures," he said. "There is a strong possibility that those considering opening a sixth form will be very carefully considering what their position is once the reality of the funding situation hits them.
"We believe this is a major issue," Mr Trobe added. "Funding for 16- to 19-year-olds is set at far too low a level to allow not only school sixth forms but also colleges to provide the quality of education they want to provide."
The ASCL is leading a consortium of organisations - including the IAA, the AoC, the Sixth Form Colleges' Association (SFCA), the Grammar School Heads Association, the Principals' Professional Council, and the Freedom and Autonomy for Schools National Association - in calling on the government to increase the budget for post-16 education, which is currently 22 per cent lower than for pupils aged 5-16.
David Igoe, chief executive of the SFCA, said that post-16 funding was "significantly below" every other phase of education and that his members would be expected to pick up the pieces as closures continued.
"It's inevitable that more school sixth forms will close," he added. "But perhaps it could be the salvation of sixth-form colleges, if they can survive long enough to rescue these kids and once student numbers begin to grow again."
Robin Ghurbhurun, chief executive and principal of Richmond upon Thames College in south-west London, said several nearby schools that opened sixth forms last year were already struggling to make ends meet. "Some are already facing problems, with a percentage of them at risk of closing," he added. "Many colleagues across the country have said the same: where new school sixth forms have opened, they are now struggling.
"It is undeniable that more will close, and that is a travesty for the students who will have their education disrupted."
A Department for Education spokesperson said it had already ended the "historic and unfair" difference between school and college funding, and would maintain current rates for 2015-16 so that institutions "can plan their future offers for students".
"The funding is sufficient for every full-time student to undertake a full timetable of courses. It is for schools to set their own budgets, taking into account our national funding formula, which ensures pupils on the same courses are funded equally, no matter where they study," the spokesperson added.
`It's right for schools to have a sixth form'
Jeremy Rowe, headteacher of Sir John Leman High School in Suffolk, has managed to make a success of his school's sixth form despite stiff competition from other providers in the local area.
He says the benefits of having a sixth form are immeasurable.
"The big thing is, it's hugely aspirational for the youngsters in the school," he says. "You can't put a price on that kind of inspiration, having sixth-formers around for the younger pupils to look up to.
"But the only way schools can do it is to subsidise it, and that means having bigger class sizes lower down the school. I think it's right for schools to have a sixth form, but you really have to believe in having one."