Schools with sixth-form cohorts of 50 or fewer could struggle to deliver the new wider curriculum composed of five AS-levels in the first year, leading to three A-levels, according to analysis by the Association of Colleges.
The association calculates that schools of that size would only be able to offer 10 to 12 subjects, severely restricting subject choice and combinations of students.
Under the association's proposals, pupils could be bussed in to the local college or lecturers drafted into schools to deliver lessons in shortage subjects.
The AOC solution could help to resolve the Government's political problem over what to do with school sixth forms in its post-16 review. The White Paper is now due to be published on 29 June. It wants common funding for sixth forms and FE colleges, but this might threaten the viability of the former. There would be huge public opposition to the closures of sixth forms.
In a paper presented to college chiefs at a conference in Cambridge this week, the association argues: "There should be an entitlement for young people to a full curriculum offer and providers should expect to make a full programme available, if necessary through co-operative arrangements with other providers."
The new, expanded curriculum will place added demands on learners, the report says, especially the majority of 16 and 17- year-olds who have part-time jobs.
"A more demanding learning programme may make paid employment more difficult for many, and present young people with difficult choices - especially those from poorer families - unless alternative sources of financial support while studying are available."
Without enhanced student support or the universal availability of the education maintenance allowances, students are unlikely to commit to the full programme.
The AOC model suggests that teaching time is likely to increase by about a third and group sizes could increase in a medium-sized FE college from 15 to 17. But FE colleges would present by far the cheapest way of delivering the new syllabus, with an average cost of pound;2,400 per student per annum compared with pound;2,900 for schools and more than pound;5,000 for small sixth forms.
"If FE sector resourcing was lifted to the level currently available to school sixth forms - a goal long advocated by the association - it would be possible to deliver in full the enhanced curriculum now sought by ministers."
John Brennan, the AOC's director of development, said: "What the analysis shows is that small units can't provide the width of curriculum options that young people should have access to.
"We are not suggesting that you close these institutions down - we have to find ways of widening what individual institutions can offer.
"The best way to do that is for institutions to get together and try to organise programmes on a local level. That might involve students travelling into college or could involve teachers going between sites."