Sixth forms with fewer than 80 students - about a third of the total number - have difficulty in offering enough choice or in providing value for money, says a school inspectors' report published yesterday.
Schools offering both A-levels and advanced vocational qualifications (GNVQs) may need sixth forms of at least 125, Her Majesty's Inspectors say.
In some cases, money for pupils on GCSE courses is being squeezed so that schools can support a sixth form, says the report on 110 schools from the Office for Standards in Education.
About two-thirds of the sixth forms were providing value for money, but no school with a sixth form of fewer than 100 pupils was judged cost-effective.
David West, head of the post-compulsory education team, said: "A school needs 80 students to provide 12 A-levels. That is the minimum provision one could make to offer a fair and reasonable choice."
Eight per cent of schools have sixth forms of fewer than 50, and 22 per cent have a total roll of between 50 and 100. About 4 per cent have more than 300 students.
Mr West said he hoped Sir Ron Dearing, who is reviewing post-16 education for the Government, would offer guidance on what should be provided and on sixth form size.
The inspectors found that the number of subjects being taught ranged from 31 (in a consortium) to five. The average was 17. Teaching standards in sixth forms are said to be higher than those for 5- to 16-year-olds. Standards were satisfactory or better in 93 per cent of lessons.Reuse content