No school has organised this. Schools don't promote or endorse marching or other acts of civil disobedience for any cause. In this case, it would be a safeguarding matter.
The march is organised by organisers who may or not be students, and the details are circulated on socials. Children either bunk off school or attend with their parents' blessing, taking a day off.
DC's friends went the other week, just didn't go in to school in the morning.