If I recall correctly it was the discussion on the size of Baffin Island, which looks almost 2x larger than Madagascar on the Mercantile map, when Madagascar is in fact just over 10% larger than Baffin.
Also, that we have islands nobody gives a 2nd thought to that are the size of Spain! :)
Sounds like you had a good education my lucky friend and I hope it serves you well. I think the closest I saw was historical map putting Britain at the center of a big red empire before skipping several hundred years to cover Vietnam because the syllabus said we had to.
So personally, I don't think I ever saw a geography teacher carry around a globe, nor talk about projections. I feel like I learned about it reading XKCD 10 years later.
This is true after 4th grade, but 1st through 4th usually have kids learning every regular subject from a single teacher in a single classroom. Of course there are many states and even more school districts, and it's very possible that some of them do it differently.
Looking at the syllabus for my state now (https://www.lehrplanplus.bayern.de/fachlehrplan/grundschule/...), the most advanced world map-related topic in elementary school is something like
Students
- learn how to read scale on a map
- understand where their home is on a map and in which state and country and on which continent it is.
Using their crappy search function, I don't get any results for map projections, Mercator, etc. at all, even further down the road.
(Note: I'm aware that Bavaria is known for being very centered on itself. This might be a large cause for this.)
But I don't think that's the age to learn about map projections. My oldest child just finished first grade though, so I'll keep an eye open.
Somewhere in there we learned about projections (many were the “split globe” ones).
Try to buy a world map for a classroom. Search things like "world map for classroom", "large world map", "world map poster" in Amazon. Probably 95% of the maps returned for these search terms are Mercator, but there are a few that aren't. American elementary school teachers are generalists, they teach geography without having specialized knowledge of geography, much less map projections. If such a teacher is buying a world map for their classroom, they're very likely to pick a Mercator map simply because that's what most classroom sized maps use.
(Despite this, American children are not raised to think that Canada is powerful...)