School leaders have warned that nationwide pupil protests over climate change this Friday could have an "extremely disruptive" impact on schools.
The protests are being organised by The Youth Strike 4 Climate movement, which is encouraging young people to absent themselves from the classroom over the "lack of government action" to combat climate change.
Nearly 40 protest events are said to be planned in towns and cities across the country.
But Geoff Barton, general secretary of the Association of School and College Leaders (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 on Friday.
"It is extremely disruptive for a school to have to deal with unauthorised absences and pupils will be missing out on important learning time."
The NAHT heads' union initially said it supported the action and "applauded" older pupils "making an informed decision".
Pupils 'shouldn't miss lessons'
However, today it changed its emphasis with a senior source saying: “We absolutely do not condone children and young people missing school.
“We accept that these things happen but unfortunately the statement that went out didn’t reflect our position and shouldn’t have been provided. Nothing is more important than a child’s education.
"Whilst school leaders are best placed to decide whether to agree to individual requests to attend Friday’s event, children should not be missing school unless it is in exceptional circumstances.
"There are plenty of ways that children can engage in social issues in school and without them missing lessons.”
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 in January, she warned the global elite that the "house is on fire".
The UK Student Climate Network (UKSCN), a student-led organisation focused on promoting the views of those under 18, is helping to host the UK's first Youth Strike 4 Climate protest.
On its website, UKSCN said it was not encouraging students to break the law, emphasising that permission for absence is needed from parents and schools.
Parents are able to download a letter template to send to headteachers saying they give permission for their child not to attend school on Friday.
A Department for Education spokeswoman said the planned strike was a matter for individual schools.
She added: "However, we are clear that pupils can only take term-time leave in exceptional circumstances, and where this leave has been authorised by the headteacher."