There is evidence too that academic standards in this sector are high. Ofsted’s latest sixth-form survey, Effective Sixth Forms (June 1996), indicates that 93 per cent of A-level lessons seen were “satisfactory or better”. It also suggests that sixth forms with at least 100 students are likely to provide an effective education, even though 150 had previously been regarded as the minimum figure. And sixth forms are not, apparently, expensive. The Department for Education and Employment claims that the cost to the nation of successfully completed A-levels is “broadly similar” to the cost in further education.

So it looks as though sixth forms are set to prosper. Yet uncertainties remain, especially over these questions of size, effectiveness and cost. Many sixth forms have well below 100 students; a significant number have nearer 50. How effective are such establishments?

Bill Laidler, at Aylesford GM School in Kent, is one of many who believe that a small-scale sixth form is no barrier to success. He has only 15 year 12 students on A-level courses (maths, English, geography, history, biology and PE) and 45 on GNVQ Intermediate (business, art, leisure and tourism, health and social care). “Remember,” he says, “that these are students who didn’t make it to the grammar schools. But they are confident here and do well. We offer them structure, security, good pastoral support; we are responding to the market. ”

Jan Mitchell, head of the LEA Sir William Nottidge Technology School in Whitstable, Kent, agrees. “A small but effective sixth form is viable and cost-effective,” she says. “It offers choice, addresses specific needs and is very much what the local community wants and the national education targets require.”

Small may be beautiful to these heads, but nationally there are many complicating factors: competition from further education, for example, and the ability range and social composition of the school’s intake. Funding is also a major preoccupation. Even schools with large sixth forms are finding it increasingly difficult to offer the breadth of subjects or the individual student support and monitoring they regard as essential.

Howard Green, principal of Eggbuckland Community College, Plymouth, has a sixth form of 300. But he relies on the Tamar Valley consortium of schools and colleges when it comes to GNVQs and NVQs, with students dividing their time between school (or college) and work placement. “Without the consortium, ” he says, “we would be under pressure to raise class sizes, or to reduce contact time.” Already, there are A-level groups of up to 29 - the sort of “saving” that isn’t open, as he points out, to small sixth forms.

What focuses the issue of costs so sharply is the prospect, clearly signalled by the DFEE, of a move away from enrolment-based funding to the sort of system that applies in further education, with a tariff based on the individual student’s programme and progress.

Such a system appeals to Howard Green and some other Devon heads, who clearly feel that their authority is at the wrong end of a national spending curve which ranges from about Pounds 2,000 per sixth form student to more than Pounds 2,500. It would, however, put pressure on smaller sixth forms. In East Sussex it has been calculated that the cost of a three-A-level programme ranges from Pounds 2,172 per year where group sizes average 14, to Pounds 3,174 per year when they drop to eight. Which of these extremes, asks county education officer Michael Nix, are unit values more likely to reflect?

Unit funding, as he acknowledges, could be a powerful instrument for improvement. The tariff could be calculated, for instance, to reflect the value added; additional studies; the quality of pastoral care; and inter-school collaboration. It could do more than Dearing to make curriculum breadth a reality. But as Howard Green points out, units could equally well ignore such things.

What is clear is that any new funding system is unlikely to change the mounting political pressure for a switch in resourcing from post-16 to primary education. In some respects, sixth forms may have been too successful. “We’re riding high,” one head reports, “but I’ve got a feeling that the crunch is just around the corner.”