At least in the US plenty of schools in hot climates do not have A/C.

https://www.the74million.org/article/exclusive-too-hot-to-le...

Even those that do have A/C don't cancel class when it breaks.

https://www.floridatoday.com/story/news/education/2019/09/11...

The real takeaway from the 74 Million piece is that information about HVAC in American schools is difficult to come by. It would certainly be interesting to look at COVID-19 in schools vis-a-vis A/C, but good luck prying information from the school districts about either infections or air conditioning. Perhaps it would be easier in a tropical country like SG where A/C is more of a necessity than a luxury.

Most public schools in Singapore don't have A/C in most of their classrooms.