Pupils in the West Midlands are being crammed into classes of 45 to ensure teachers are given time out for marking and preparing lessons, a union has warned.
The National Union of Teachers says primary schools are splitting classes up and sharing pupils between two others in a bid to adhere to new laws relieving teachers from teaching duties.
With a typical primary class containing 30 children, it means teachers having to cope with an extra 15 pupils during a session, the union claims.
"I have had three or four instances of this brought to my attention," said Brian Carter, Midland regional secretary of the NUT.
"We have got some reports of pockets throughout the Midlands. What we are trying to do is alert parents to the situation and say to the schools is this the best way of resolving the need to give control time to teachers?"
Under the Government's schools' workforce reforms, teachers were guaranteed ten per cent non-teaching time to prepare and plan lessons and catch up with marking from September. Extra money has been pumped into schools which Ministers claim will pay for teaching cover.
But according to the NUT, some cash- strapped headteachers have " cut corners" by dividing up classes instead of employing additional qualified teachers.
"What is happening is where you have three classes in the morning the third class is being split between the other two classes, which gives time off for the teacher for their planning, preparation and assessment time," said Mr Carter.
"That is the kind of thing that might develop when schools are faced with budgetary problems and the requirement to provide ten per cent of non-contact time."
Earlier this year Schools Minister Jacqui Smith was accused of giving headteachers the green light to double class sizes to implement the reforms.
It followed claims by the Professional Association of Teachers that hard-up heads were considering doubling class sizes and using teaching assistants to cover classes.
Speaking after the conference, Ms Smith told reporters headteachers should run classes of up to 60 children "if they think that's right for their schools".
But the Labour MP for Redditch later denied that was what she meant.
Mr Carter claimed evidence of a dramatic decrease in the use of supply teachers showed schools were increasingly splitting classes and relying on non-qualified classroom assistants.
"We have supply teachers in the Midlands who complain they can't get work," he said.
" They can't get work because a lot of short-term supply work is being taken on by classroom assistants who aren't taking children further, just managing them."
Mr Carter said such problems occurred because schools lacked clear guidance on how to implement the reforms.
" The Government is ambivalent on how schools provide PPA time and how you cover for short-term absence," he said.
"There is no clear definition what a short-term absence is. We are quite clear that what we want is cover supervisors only to be used in emergencies while you find a supply teacher."
As well as the ten per cent non- teaching guarantee, teachers have also been freed up from duties such as photocopying by increased use of classroom assistants.
A spokesman for the Department for Education and Skills said: "There is more than enough money to support PPA and more than 99.5 per cent of schools are implementing it."