Schoolchildren across England are "on strike" in protest at controversial tests for six and seven-year-olds.
The action comes after more than 40,000 people signed a petition supporting a boycott of Year 2 Sats by teachers.
The Let Our Kids Be Kids campaign has organised the day of action in protest at children being "over-tested, over-worked and in a school system that places more importance on test results and league tables than children's happiness and joy of learning".
The petition added: "We want our kids to be kids again and enjoy learning for learning's sake, not for Ofsted results or league table figures. Bring back the creativity and the fun - say goodbye to repetition and boredom."
Sats are taken by children aged six or seven in Year Two and then again in Year Six, aged 10 or 11, before a third set in Year Nine, aged 13 or 14.
Parents and children are organising fun and educational activities during the protest all over the country.
Ben Ramalingam from Brighton - where lots of concerned parents are meeting in Preston Park - is keeping his five-year-old son off school in protest at what he says is a hothouse culture that has made young children stressed.
He said: "Our kids are being left disengaged and stressed. Kids who previously loved school are now refusing to go.
"There are a number of people who are saying this has the potential to turn into not just an educational crisis, but a mental health crisis."
Education Secretary Nicky Morgan warned that missing school even for a single day would be "harmful" and called for those behind the "damaging" campaign to reconsider.
In a speech on Saturday, she said: "To those who say we should let our children be creative, imaginative and happy - of course I agree, both as a parent and as the Education Secretary.
"But I would ask them this: how creative can a child be if they struggle to understand the words on the page in front of them? They certainly can't enjoy them.
"What are the limits placed on a child's imagination when they cannot write down their ideas for others to read?"