There have been some classroom studies in the UK showing a reduction in illness as well. They were a bit better run than this because they had some some pupils in filtered air and some not and even with the mixing at break time the reduction of illness met clinical significance. Given Covid is doing its thing all year around its quite an easy thing to do to drastically reduce the time teachers and pupils spend away from school and its a fairly easy measure to adopt. They do need to have the filters running however and that will involve explaining to the teachers how they work and why they are needed because in a lot of studies compliance has been a big issue usually due to misinformation rather than issues with the devices.