Children as young as five are due to be among the thousands of pupils walking out of their lessons as part of a climate change protest.
Primary school children, as well as teenagers and university students, are being encouraged to take part in a mass “strike” on Friday over the "lack of government action" on the environment.
More than 40 protest events are said to have been planned in towns and cities as part of the UK’s first Youth Strike 4 Climate. It comes as similar protests have happened around the world.
Anna Taylor, 17, who is one of the organisers of the protest, told the Today programme they were expecting young children with their parents to be among the thousands of projected participants.
She said: “It is important to remember that simply learning about it in school has not done enough. We have tried to learn about it in school and yet we are still heading towards an immense climate catastrophe which will end the life of loads of species on earth.”
But it leaves headteachers in a difficult situation as they are expected to ensure children attend school unless they are ill. Their absence would have to be recorded as “unauthorised” in many cases.
Anna said her school had threatened to give her a detention and an unauthorised absence notice if she does not come in because of “safeguarding” concerns.
However, a spokesman for the National Association of Head Teachers (NAHT) said the union “applauded” older pupils “making an informed decision”.
He added: “Society makes leaps forward when people are prepared to take action. Schools encourage students to develop a wider understanding of the world around them.
“A day of activity like this could be an important and valuable life experience.”
But the Association of School and College Leaders (ASCL) has warned that the protests will have an "extremely disruptive" impact on schools as pupils will miss out on learning time.
Geoff Barton, general secretary of the ASCL, said: "While we understand the strength of feeling over the very important issue of climate change we would urge pupils against walking out of school.”
The nationwide strike is inspired by teenager Greta Thunberg who protests every Friday outside Sweden's parliament to urge leaders to tackle climate change.
At the World Economic Forum in Davos, she warned the global elite that the "house is on fire".
She said: "I want you to panic. I want you to feel the fear I feel every day. We owe it to the young people, to give them hope.”
The Department for Education said that whether to allow children to take part was a matter for individual schools to decide.
