Yeah it was a middle school thing in Finland too, at least in the 90’s.
I did an exchange year in the US in my 2nd high school year, and I was honestly a bit surprised at how… well, simple it all was. I was a senior in the US and I’d learned just about everything they taught that wasn’t specific to the US or the English language (and even some of those…) either in my 1st year in high school or in middle school.
It’s the step between primary and secondary school that a lot of countries have, also known as intermediate school, junior high school, junior secondary school, or lower secondary school: https://en.wikipedia.org/wiki/Middle_school