I would really like to know which parts of the world use Mercator projection on their standard school wall maps.

At least in Germany (and both former East and West) it certainly isn't in use since at least the 1950s (and probably much earlier), but instead a projection type that maps land masses closer to their actual relative sizes. Mercator is extremely rare and only used in very specialized professional maps (e.g. for navigation), but certainly not as the popular map projection in education.

Also don't schools use physical globes anymore?

This whole drama sounds like it's about a century late to the party tbh.