GCSE pupils do better at schools without sixth forms because "excessive resources" tend to be allocated to post-16 students, according to research from Cardiff university.
A study of school size and its effect on achievement, by Professor James Foreman-Peck, found that "sixth-forms in Wales are not the unambiguous educational advantage for the 11 to 16-year-old group that is sometimes assumed".
A comparison of GCSE results showed that a higher percentage of pupils achieve at least five A*-C grades at schools with sixth forms than those without (48.5 to 41.5 per cent). But when Professor Foreman-Peck included factors such as the number of pupils with free school meals and the "value added" between key stage 3 and GCSE he found that "schools with sixth-forms add less value to those below the sixth than schools without".
He concluded: "This is consistent with schools starving GCSE pupils of resources in order to nourish sixth-formers. Schools without sixth forms tended to perform less well in absolute terms, but they also enrolled a higher proportion of deprived pupils."
Professor Foreman-Peck, director of the university's Welsh Institute for Research in Economics and Development, also reported that the optimum size for a secondary school is 600 pupils.
"Schools of more than 600 pupils tend to achieve poorer public exam results in the long run, and a higher proportion fail to gain any qualifications," he said.
Contrary to the view of the Audit Commission (which carried out a study in England in 2002), he said: "There is no reason to suppose educationally-efficient schools should be larger than the present average size for Wales."
He suggests large schools should be reduced to 600 pupils at most, including sixth-formers, and to 500 for 11-16 institutions. In 2003-4, the average Welsh secondary had 950 pupils, and 86 out of 227 had fewer than 800 pupils.
Alun Griffiths, head of Cefn Saeson school in Cimla, Neath, an 11-16 comprehensive with 850 pupils, said: "I feel we are at the optimum size, with good communication between pupils, teachers, parents and governors. We can focus our staffing and resources on our age group."