Sometime within the past month or so, many international schools across Thailand no longer allow people to read or post reviews for schools. It's not for every school, but many schools I read reviews for no longer have review ability. Perhaps schools that have bad reviews no longer allow reviews. "Posting is currently turned off."
Do you think this is permanent? What is the best resource for checking reviews for schools?

I seriously doubt if a few schools in Thailand have the ability to turn off google reviews for themselves.

Obviously it wasn't individual schools deciding to do so.

Have you even tried checking recently? Check Bangkok Prep for example

Perhaps there has been some pressure on google by govt in general to stop allowing "defamation" on a large scale via Maps.