The lord mayor of Sheffield has promised to defend any pupils facing punishment for skipping school on Friday to join a nationwide climate change protest.

Prime minister Theresa May and some teaching unions warned young people against walking out of class on Friday to demand action on the environment.

But councillor Magid Magid, the 29-year-old Green Party politician who became Sheffield’s youngest ever lord mayor last year, pledged to “have a word” with any teachers taking disciplinary action over the strike.

He tweeted: “If any of your teachers threaten you with some sort of action or give you any hassle for going on strike. Let me know & I’ll have a word, even if I have to come into your school!”

Responding to online criticism of teenage activists, the lord mayor added: “The old stale establishment can’t stand the political participation and awareness of young people, who understand the world better and have a greater sense of priorities than they ever will.”

Climate strike: See the best signs from the protest here Show all 29 1 /29 Climate strike: See the best signs from the protest here Climate strike: See the best signs from the protest here The protest in Parliament Square, London on February 15 AFP/Getty Climate strike: See the best signs from the protest here The protest in Parliament Square, London on February 15 Getty Climate strike: See the best signs from the protest here The protest in Parliament Square, London on February 15 EPA Climate strike: See the best signs from the protest here The protest in Manchester on February 15 Getty Climate strike: See the best signs from the protest here The protest in Manchester on February 15 Getty Climate strike: See the best signs from the protest here The protest in Parliament Square, London on February 15 Getty Climate strike: See the best signs from the protest here The protest in Brighton on February 15 Getty Climate strike: See the best signs from the protest here The protest in Parliament Square, London on February 15 AFP/Getty Climate strike: See the best signs from the protest here The protest in Parliament Square, London on February 15 AFP/Getty Climate strike: See the best signs from the protest here The protest in Parliament Square, London on February 15 Reuters Climate strike: See the best signs from the protest here The protest in Parliament Square, London on February 15 PA Climate strike: See the best signs from the protest here The protest in Parliament Square, London on February 15 PA Climate strike: See the best signs from the protest here The protest in Parliament Square, London on February 15 PA Climate strike: See the best signs from the protest here The protest in Parliament Square, London on February 15 PA Climate strike: See the best signs from the protest here The protest in Parliament Square, London on February 15 PA Climate strike: See the best signs from the protest here The march in Brussels, February 14 Reuters Climate strike: See the best signs from the protest here The march in Brussels, February 14 Reuters Climate strike: See the best signs from the protest here The march in Brussels, February 14 AP Climate strike: See the best signs from the protest here The march in Brussels, February 14 AP Climate strike: See the best signs from the protest here The march in Brussels, February 14 Reuters Climate strike: See the best signs from the protest here The march in Brussels, February 14 AP Climate strike: See the best signs from the protest here The march in Brussels, February 14 AP Climate strike: See the best signs from the protest here The march in Brussels, February 14 Reuters Climate strike: See the best signs from the protest here The march in Brussels, February 14 Reuters Climate strike: See the best signs from the protest here The march in Brussels, February 14 Reuters Climate strike: See the best signs from the protest here The march in Brussels, February 14 Reuters Climate strike: See the best signs from the protest here The march in Brussels, February 14 Reuters Climate strike: See the best signs from the protest here The march in Brussels, February 14 Reuters Climate strike: See the best signs from the protest here The march in Brussels, February 14 AP

Thousands of schoolchildren took part in the Youth Strike 4 Climate day from 11am on Friday. Action has been taking place in 60 towns and cities across the country with youngsters carrying banners bearing slogans such as: “There is no planet B” and “When did the children become the adults?”

Demonstrators in London sat on the roads around Parliament and chanted “we’re not moving” as they created a human blockade.

In Belfast, a crowd of young people gathered outside City Hall. Maia Willis Reddick, a 17-year-old student at Belfast’s Methodist College, said her school had been supportive of her joining the action.

She said: “We are still very conscious about school. I have A-levels, I have stuff to do, we just want to make the point that we are willing to take drastic action in order to highlight the problems of climate change.”

However, the strikes were not welcomed by some school leaders and the education secretary Damian Hinds, who said missing class was not the answer.

Geoff Barton, general secretary of the Association of School and College Leaders (ASCL), said the strike was “not the right approach”.

Student attending protest in Parliament Square (EPA)

Mr Barton explained: “The problem is not only the disruption and loss of learning time but the fact that schools have responsibility for the safety and wellbeing of pupils, and it is a serious concern if pupils are off site and unsupervised.

“We appeal to all involved in the protests not to repeat the exercise, particularly as many students will soon be taking exams which are crucial to their life chances.”

Downing Street said while it was important for young people to engage with issues such as climate change, the disruption to planned lesson time was damaging.

A No 10 spokeswoman said it was “important to emphasise that disruption increases teachers’ workloads and wastes lesson time that teachers have carefully prepared for”.

But the demonstrators did gain support from Labour leader Jeremy Corbyn and Scotland’s first minister, Nicola Sturgeon, who described the pupils’ actions as a “cause for optimism in an often dark world”.