When I was a kid in Los Angeles, the public school system had "junior high schools" for seventh to ninth graders. Today, the same system has "middle schools" for sixth to eighth graders. For example, Millikan in Sherman Oaks, where I went to summer school in the early 1970s, was a junior high school back then, but when my son attended it, it was now a middle school. 

I've never seen an explanation for this change.

Perhaps junior high schools were intended for an earlier age when finishing 9th grade was considered adequate?

I have to say I think the new system may be better. I attended a genteel Catholic school from first to eighth grade and then a Catholic all boys high school, and had a nice time at both. I didn't like summer school at the public junior high school, though. Looking back, the social scene was immaturely sexualized. 

So, it may make sense to take ninth graders, who are feeling their oats, away from being top dogs in junior high school and instead make them the bottom of the pecking order in high school.