School leaders are having to wrestle with their consciences over pupils joining the nationwide climate strike to be held on Friday afternoon, caught between their duties as teachers and instincts as educators.

Thousands of the more than 8 million school pupils in the UK are expected to walk out of lessons to show their concern about the threat of escalating climate change.

Layla Moran, the Liberal Democrat education spokeswoman and a former science teacher, said she will be joining a rally in Oxford and urged school leaders to accommodate pupils who join the protests.

“As a former teacher I absolutely understand the frustrations that teachers and schools may have with these strikes. However, I would describe this as a teachable moment,” said Moran, the MP for Oxford West and Abingdon.

“I hope schools, colleges and universities see this in the positive light it is meant and equally hope those students act sensibly with making sure adults know where they are and making up the missed work.”

But the National Association of Head Teachers, which last week offered some support for those joining the strike, gave a more hard-headed assessment to its members as the day of the action approached.

“Pupils should only be out of school in exceptional circumstances. Whilst NAHT supports the right of young people to express themselves, first and foremost, pupils should be in school during term time,” the union said.

“While a school leader’s role is to ensure children attend school, are kept safe and receive a good quality of education, it is right that individual school leaders can decide how best to respond to any proposed protest by students in their school on Friday.”

Chris Keates, general secretary of the NASUWT teaching union, said young people were right to be concerned about climate change but warned that teachers had genuine safeguarding issues to consider.

“It’s not appropriate for pupils to just walk out of school. The young people organising this are potentially putting themselves and others at risk by simply walking out of school,” Keates said.

“Teachers take their responsibility for pupils welfare during the school day seriously and while they may have sympathy for the cause will not be able to condone pupils just walking out.

“A well-planned, accessible demonstration of young people at a weekend or during a holiday would enable thousands to participate and be a powerful protest.”

The Department for Education’s guidance says unauthorised absences must be recorded as part of safeguarding, but gives room for headteachers to approve absences in “exceptional circumstances”.

The organisers have written form letters for the parents of those wanting to take part, which argue that protesting qualifies as civic engagement and climate change is an exceptional circumstance.

With the strike taking place on the final day before half-term holidays in many parts of England, some schools may adopt a relaxed attitude. But others have already informed their students any absence will be noted as unauthorised.

Geoff Barton, a former secondary school headteacher who now leads the Association of School and College Leaders, 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 day of action follows the example of Greta Thunberg, a 15-year-old Swedish schoolgirl who began protesting outside her country’s parliament in August. Her solo effort has since grown and spread to countries around the world including Germany and Australia.

In a letter to the Guardian, more than 200 UK academics said they offered “full support” to the school strikers. “They have every right to be angry about the future that we shall bequeath to them, if proportionate and urgent action is not taken,” it said.