There are lots of online events happening throughout the day, mentioned on the Clean Air Day website. Clean Air Day is an award-winning annual campaign coordinated by Global Action Plan which has provided a School Social Media Pack. There is an assembly pack, letters to school leaders, along with KS1 and KS2 resources that can be downloaded and used in school to help children to understand the issues and what they can do to help. There is also a pledge card, this is a great idea; however, I would create them on cards shaped like clouds, trees, or flowers and hang them around the room, or in the windows for show.
The air pollution leaflet could be distributed to families by email or on social media and the clear air posters could be posted around school to draw attention to the event.
The poster explaining the effect that pollution has on the body is useful for science when studying the human body too.
There is information on creating walking buses, planning a walk to school week and more. The site has lots of information for use in schools, even if you do not celebrate the actual day on Thursday. Just follow this link to all of the resources.