Parents in Kent have accused the county council of undermining their children's chances of passing the 11-plus.
They said that the exam required the use of calculators which the council forbids. Some questions in the maths paper were so hard that the pupils were in tears. One was sick during the test and another suffered a stress-induced nose-bleed.
"I'm upset with the council and so are a lot of other parents," said Dawn Boughton of Tonbridge, whose daughter Rachel, 10, is waiting to hear if she has won a place at one of Kent's 39 grammar schools.
"Rachel was clearly distressed when she could not complete the maths paper as were children in other schools. Many will not have performed as well as they should."
She said this year's exam was far more difficult than the one taken by her son David, 12, who now goes to a grammar school.
"Most of the questions this year seemed to be about algebra and measuring areas. I would expect an 11-year-old to be able to do areas, but the algebra was geared more towards GCSE candidates. At least we should have an apology from the county council," she added.
Sheila Webster, acting head of St Stephen's primary school, Tonbridge, which Rachel attends, said schools were told only last November that the maths paper had been changed to multiple choice so there was little time to prepare. Pupils sat the test in January and will get their results in March.
She said she would have needed a calculator to answer a number of the questions. "It was traumatic for the children and a lot were quite distressed. I think it will have affected the performance of some."
Andy Blundell, head of St John's C of E primary school, Tunbridge Wells, said the maths results across the county would be lower this year because the exam was so hard but, as all children sat the same papers, the outcome would not be affected. However, the exercise had been unnecessarily stressful.
According to a county council spokeswoman, the English and maths tests, which had been standardised by the National Foundation for Educational Research, were designed to test what the above-average child could reasonably be expected to do.
"Algebra questions have been part of the maths test every year and there were no questions for which an above-average child would have needed a calculator, " she said. This year, about half of Kent's 14,600 10 and 11-year- olds sat the 11-plus, about one quarter of whom will pass.