I have a child at one of Chiang Mai's international schools and they seem to pay not a blind bit of attention to this health hazard until today, when they stopped some outdoor activity. (One of the parents even distributed masks to the children, because the school appeared not to care.)

Has your school issued any warnings or done anything to try to mitigate this health hazard?

If it's the 5 baht cloth 'medical' masks then it's pretty useless against tiny dust particles.

I have a feeling that for it to be effective, it would make breathing a lot harder, so not comfortable for long term use. But I'm not sure, perhaps someone with more experience can shed light on to exaclty what type of mask/filter would be needed. I guess something that blocks vapours / pesticide fumes would do?

When do the international schools shut?

I guess there is a reason that Thai schools are shut at this time of year.