A leading education adviser says class sizes of up to 80 could help some schools.
David Carter, head of the National Remodelling Team advising on teachers' workloads, says big classes release staff to deal with children who need more attention.
Mr Carter is head teacher at Deer Park School in Cirencester, Gloucestershire.
The National Union of Teachers (NUT) has criticised the idea.
GCSEs
Deer Park school has merged certain classes together over the last six years, particularly in subjects such as English and Humanities.
Since the introduction of the larger classes, its GCSE results are believed to have improved markedly.
John Bangs of the NUT told the BBC: "There is no evidence that class sizes of 80 raises standards.
"What does raise standards is targeted, effective teaching that gets at pupils' needs however they learn, and whatever their ability."