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.
"Schools think that this is going to give them an edge in getting the results they need - so that's the driver," Mr Northcott said at the union's annual conference in Belfast.
"It seems like an ill-conceived response to this pressure."
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.
"I think children would be better off in the Easter holidays, absolutely, if they have been set some homework and if that homework is useful and productive, they should be doing that.
"But they should also be doing enjoyable, engaging things in their own time, with their own friends, spending time with their families, which is all a critical part of a healthy childhood."
General secretary Chris Keates said: "The growing trend of Easter Sats classes in primary schools is a worrying reflection of the high-stakes accountability regime they operate in.
"Children should be spending Easter with their families and friends, not cramming for Sats."
Shadow education secretary Angela Rayner said: "Our pupils are the most tested in the world, but there is no evidence that the current high-stakes testing regime improves teaching and learning."
But Mr Hinds said exam stress at primary school level was not inevitable.
"All over the world, schools guide children through tests without them feeling pressurised.
"These are tests of our education system, not our children.
"No-one has ever been asked for their Sats results when they go to a job interview - why? Because they are not public exams."
Labour 'would ban Sats'
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.