There was a leak at a methane storage facility in the LA area. Several schools were provided air filters well after the event (methane is lighter than air). They had the filters for four months even though the methane levels were normal. Test scores increased.
edit - It was not easy find this. As I wasn't in a hurry at the time I took a while to find an acceptable model. Thinking more about it using eBay's boolean search options may have helped drill down to good choices.
Perhaps dust and particulates? The SoCal area is very dusty. If you leave your car parked for a few days it will have a fine layer of dust on it.
Imagine breathing that in along with all the other germs normally present in a classroom. Less sickness should allow more learning.