Parents organise outdoor learning events for Tuesday 3 May in protest at testing regime.
Campaigners say that thousands of parents have expressed an interest in keeping their children at home for a day in protest at the current assessment system in primary schools.
The government's introduction of 'tougher' tests for 7- and 11-year-olds has already sparked criticism from teaching unions for delays and a lack of clarity on what should be done.
Now the Let Kids Be Kids campaign group has said that it is “overwhelmed” with the level of support from parents who have said they will take their children out of school for the day on Tuesday (3 May).
As TES reported earlier this month, the movement began with a group of Year 2 parents who were concerned about the tests for 7-year-olds, but it has since been taken up by parents with children in other year groups who are also concerned about testing.
A spokesperson, who wanted to remain anonymous, said: “We want people to tell schools why they are doing this. We want them to explain why their child is absent and ask that the headteacher endorse it as an exceptional circumstance.
“We want schools to know that this isn’t an action against them. It’s against the national policy. We want schools to be fully aware this is a campaign that is supportive of teachers and headteachers, so they know there is this massive parental surge of enthusiasm for the cancellation of Sats.
“We don’t want parents to do this and not be counted,” she added. Parents taking part in the boycott are using the group’s website and Facebook to organise outings such as forest school activities in parks, visiting a wildlife sanctuary or museums.
The group has also organised a petition on the 38degrees website that calls for an end to Sats and has been signed by 27,775 people.
The NAHT heads' union did not want to comment on the boycott, but said that the online petitions showed the strength of feeling around assessment.
Kim Johnson, incoming president of the NAHT, said: “We believe that parents and teachers agree that large-scale statutory tests like Sats don’t necessarily deliver educational benefit and fail to give parents and teachers the information they need to support children as they learn. Our conversations with parents strongly suggest that they would far prefer that their children are tested on a ‘little and often’ basis to consolidate learning, rather than in high stakes end of stage assessments.
“Parents and school leaders both agree that now is the time to call for a better system of assessment – one that works for parents, pupils and teachers, rather than one that just ticks boxes for bureaucrats and politicians.”
A Department for Education spokesperson said: “We are clear that tests should not be a cause of stress for pupils – they help us ensure schools are performing well, and we know the best schools manage them successfully."