Teachers have voted to oppose "reprisals" against pupils who walk out of school to campaign against climate change.
The NEU teaching union voted to stand in "full solidarity" with those who have been taking part in global protests.
A motion calling for teachers to oppose "any reprisals" – such as detentions and exclusions – against students taking action was passed at the NEU's annual conference in Liverpool.
Background: Spare pupil protestors detentions, say teachers
Read: ‘The climate strikes threaten the safety of our pupils’
Read: 'It's short-sighted to dismiss the pupil strikes'
It states: "The rights to strike and protest are fundamental democratic rights for students and workers alike."
The motion instructs the executive: "To call on schools not to take action against students.
"We pledge to support student defence campaigns as necessary."
The amendment to a main motion on climate change also calls for it to be integrated into the curriculum.
But the vote puts the union at odds with heads' associations, and the Department for Education, which repeatedly called on pupils not to walk out of class for climate change protests.
Speaking before the conference, NEU joint general secretary Mary Bousted said: "We are delighted that students are taking the issue so seriously.
"We support the cause, absolutely. We couldn't support children walking out of school and walking out of lessons, because that would potentially put those children and young people in danger – we have a duty of care.
"So we support the cause, absolutely, but we have to say we can't support young children walking out of lessons and walking out of school."
Education secretary Damian Hinds has previously said that skipping class would not help the environment.
He said that while he wanted young people to be engaged with the issues affecting them, he emphasised this should not create extra work for teachers.
Jane Nellist, from the union's Coventry branch, said: "The millions of young people across the world, those young people engaged in these strikes, are laying down a gauntlet – enough is enough.
"As teachers, as a trade union, we need to stand in full solidarity with our students taking that strike action, we need to oppose any reprisals against them and we need to campaign to continue to campaign for these important global issues to be integrated into our curriculum."
Robert Behan, from the Newham district, said: "I have been told by my management not to talk about or support student strikes within my remit – it is a safeguarding issue.
"I am from the London borough of Newham, which is one of those which is being deeply affected by a recent spate of knife crime, which is also a safeguarding issue.
"Are we picking and choosing which issues that affect our students are going to be talked about?
"We have got to support our students. When our students are victims of the climate crisis, we should also support them on that."
Students have taken part in three protests as part of the Youth Strike 4 Climate movement in as many months, with the latest one taking place on Friday.
The mass gatherings across the country have led to roads being brought to a standstill, and children of all ages carrying banners and placards calling for action on climate change.