I have definitely noticed different states in the US separating grades differently. Where I grew up 1st through 6th is Elementary School, 7th and 8th are Junior High, and 9th through 12th are High School. But where I'm living now Elementary School is 1st through 5th, while 6th through 8th are considered Middle School. And I've even talked with a few people from other states where 8th grade is part of High School. So there is no consistency at all between states.
But even in the state I grew up in, for the larger towns, Junior High gets its own building typically. It was only combined with Elementary in my town because the town is so small. The population of my town was less than two thousand people when I was growing up there. That's also why we had no high school. High school students commuted to a school that was about 60 miles away. That was the closest place that had a high school.
I kind of wish we could have something for the Sims that allow us to customize schools for the Sims. For example, make Snowy Escape similar to the Japan type schooling. Since their school structure is different.
I would just like the schools in Sims to be less generic, and give our Sim teens more stuff to do.
I don't play with kids and teens a whole lot, but when I do, I've usually used The "kids and teens can quit school" mod by Triplis so I can set up an actively played "school" using the Get Together club system instead. I have them do activities there that actually work on skills they might need as adults. And that approach can be customized however you want through the club activities, club uniform, and what's on the lot.