There are reasonable safeguarding concerns about not knowing with certainty where children are during school time. I can see positive benefits in whole classes participating though, as part of the national curriculum with follow-up work to debate the relative merits of organised action and passive acceptance, government commitment, peaceful protest versus rioting, comparative movements in other countries, statistical analysis, intra-group dynamics, the role of leaders and followers, how democracy can be influenced by lobbyists, bladder control during Police kettle gathering tactics, fresh air and exercise, preparation and organisation, design and construction of placards, discipline in public, ... maybe even climate change itself.
I would fully support an integrated and coordinated approach to this idea.