Teaching a 60-pupil primary class so that teachers can have non-contact time is harmful to children's learning.
Keith Watson, assistant headteacher at 409-pupil Portswood primary, Southampton, was given pound;2,500 by the Department for Education and Skills to test the effect of doubling up classes, under the DfES's now defunct best practice research scholarship scheme.
Government guidance on how to provide teachers with half a day out of the classroom for planning, preparation and assessment (PPA) time includes pairing classes or taking a whole year group, where these are beneficial to the curriculum.
But the school found that pupils suffered from the consequent lack of personal attention.
Dr Watson said: "You can create activities that pupils can sit and get on with, and then police them. Secondary schools have cover supervisors.
Pupils are told to do pages 53 and 54 and are supervised, but it's not teaching. If we did that here we could manage, but it is not quality teaching."
Dr Watson taught four lessons to a Year 5 group of 60 pupils and two lessons to a doubled-up Year 2 group. Mary Longhurst, a Year 5 teacher, taught six lessons to the Year 5 group and Lara Roberts, a newly-qualified Year 5 teacher, taught three literacy lessons. Each lesson lasted one hour.
Pupils in Year 5 were taught in two adjoining classrooms with a door open between them. Year 2 pupils were taught in separate classrooms with the teacher going in and out.
The work the children produced in the lessons was examined and was not thought to be as good as usual. This was particularly true for the higher ability pupils.
Dr Watson said: "What is missing is the quality intervention between teacher and pupil - the questions you ask to assess what the pupil is learning and how to take that forward.
"You would not necessarily talk to every individual, but in a class of 30 you may have three groups and you would have a conversation with eight or nine pupils in those groups. That is now changed to a group of 19."
The school has decided to use three qualified teachers to provide PPA time for its 14 class teachers.
A TES survey found that while 23 per cent of secondary headteachers said providing PPA time would create budget problems, this rose to 65 per cent in primary schools. Plans for the next generation of schools unveiled last year included classrooms big enough to teach up to 90 pupils.
Ministers have praised schools such as Kemnal technology college, in Kent, for its pioneering use of teaching assistants. The school runs classes of up to 75 with teachers being assisted by support staff.
A study in the US has found that pupils who spent their first four years at school in a class with fewer than 17 pupils were less likely to have dropped out by age 18 than those in classes of between 22 and 26 pupils. The study, by Buffalo university, is published in the Journal of Educational Psychology.