There is a growing trend of primary schools running Easter holiday revision classes for formal tests, known as Sats, a teachers' union says.
The NASUWT union says "cramming sessions" are becoming more common in schools ahead of the tests sat in May. It says children should not be in school over the holidays, but should be spending time with their families.
Education Secretary Damian Hinds said Sats were tests of the education system in England, "not our children".
The results of Sats tests taken by 11-year-olds are published each year in primary school league tables, published by the Department for Education.
Darren Northcott, the NASUWT's national official for education, said it was the pressure of accountability that was leading schools to open up for Year 6 pupils over the holidays.
He said that while attendance at the Easter booster sessions he was aware of was voluntary, it was not clear what sort of message parents were being sent.
But Mr Hinds said exam stress at primary school level was not inevitable.
Last week, Labour leader Jeremy Corbyn announced he would scrap Sats if his party came to power, saying the move would help improve teacher recruitment and retention.
Instead, Labour would introduce alternative assessments which would be based on "the clear principle of understanding the learning needs of every child," he said.
But Schools Minister Nick Gibb said abolishing Sats would be "a retrograde step". He said the move would "keep parents in the dark" by preventing from knowing how good their child's school is at teaching maths, reading and writing.
