Sir Christopher Ball's Platform article (TES, November 4) argued that a school should be expected to offer 10 A-levels and 10 General National Vocational Qualifications.
Assuming two-year full-time courses and average group sizes of 10, he claims this requires a minimum critical mass of 400.
His sums are wrong. Students do not study a single A-level. If they take 3 A-levels each, 67 students are needed across Year 12 and 13. If we assume that a GNVQ is full-time (it is actually meant to be equivalent to 2 A-levels), his sums in this area are correct, needing 200 students. So this hypothetical situation leads to a critical mass of 267, not 400.
A much more likely situation is a combination of 15 A-levels and five GNVQs. Carrying out the same calculation here gives a critical mass of 200. This could be generated by a four-form entry school with a staying on rate of 80 per cent.
All this assumes a totally isolated sixth form with no other provision available. In practice, there are a variety of providers of post-16 courses in any locality.
Most institutions with post-16 students collaborate over their provision, despite the Government's attempts to tear them apart. The critical mass for a school sixth form collaborating with other providers will be considerably less than200.
Dr P J HUNTER Chief education officer Staffordshire.
G W A Cooper Headteacher Weston Road High School Stafford.