Not necessarily high confidence but certainly worth exploring given the minuscule costs compared to many, many other expenses, a local school district bought classroom air filters for about $100k each and had dramatically lower COVID related absences than nearby county and city school districts that didn't have them.
$100k per classroom is significantly different from "fans". That's also an extremely privileged position, because most schools refuse to budget $20 a classroom for basic supplies like whiteboard markers.
I have no clue where the $100k per classroom number came from in the parent comment(each school maybe?), A Denver based project had an installed cost of less than $800 and a yearly energy cost of $361 per classroom.
That's still significant, but good luck convincing the school board to do that instead of continuing to funnel money wherever they usually do instead of improving classrooms.