Latest figures have revealed the Northampton schools where pupil numbers are greater than the recommended levels in some classes.
Figures uncovered by the Labour Party show that 109 out of 256 (or 42 per cent) of primary schools in Northamptonshire have at least one classroom crammed with more than 30 children.
Those in Northampton that exceed the Government guideline (with the number of classes in excess in brackets) are:
All Saints CofE Primary School (1)
Woodvale Primary Academy (1)
Green Oaks Primary Academy (1)
Cedar Road Primary School (1)
St Andrew's CofE Primary School (1)
Abington Vale Primary School (3)
Whitehills Primary School (2)
Boothville Primary School (1)
Lings Primary School (1)
Standens Barn Primary School (2)
Duston Eldean (1)
Weston Favell CofE Primary School (2)
Local union representatives said that both teachers and pupils lose out when class sizes get too high.
Alan Hackett, of the NASUWT branch for Northamptonshire, said: "If there's too many pupils, then you simply can't give acceptable levels standards of one-to-one attention.
"If that's happening then teachers can't stretch the children to their full abilities."
Latest estimates show that Northampton is four primary schools short of its requirements.
Mr Hackett said the push to make schools into academies has led to headteachers gradually pushing up class sizes.
He said: "The ideal solution would be to go back to having the county council as the local education authority, which had an overview.
"With the academies, we've got almost a free-for-all."
Commenting on the national figures which show a similar trend, Shadow Education Secretary Angela Rayner pledged that her party would cap class sizes to 30 if they came to power at the next election.
She added: “This situation is totally unsustainable. If ministers really wanted to give every child the education they deserve, they wouldn’t pack so many five, six and seven year olds into classes of this size."