Cardiff council has asked schools to show common sense and leniency to pupils missing school to protest.
Up to 1,000 people, most of who are children, have taken part in a strike to protest the governments lack of action on climate change.
Council leader Huw Thomas said the children should wear their absent mark as "badges of honour".
He said: "If I was still of school-going age, then I would be on strike today.
"Whilst schools in Cardiff have been advised to mark pupils on strike as ‘Absent’, we would hope that schools show common sense and leniency with those pupils who march for the Climate – they should wear their absent marks as badges of honour."
You can follow updates of the strike here.
A spokesman for Cardiff Council added: "For the Council’s part we are determined to play a leading role within Wales in addressing climate change. Declaring a Climate Emergency was an important first step, but critically, we are following this up with action, including building a 10mw Solar Farm on Lamby Way, construction of which starts this month.
"Climate Change is the most important issue we face as a species, and in the next few months, Cardiff Council will be bringing forward a revised ‘One Planet Cardiff’ strategy to ensure Cardiff plays its full part in the solutions.”
