..or schools willing to change their air filters (or attempt to measure the effect of having changed their air filters) are more likely to be among the higher performers according to traditional KPIs (when controlled for observer bias)?
The article explains why the air filters were installed, and it doesn't fit your supposition.
agree, I actually expect there to be all sorts of positive correlations with clean air - but they all start with caring about it enough to do something like change a filter