Junior High
1. Second (of three) phases of compulsory public education in many countries, notably most of the US, Korea, and Japan. Also called "middle school" or "intermediate school". In the US, usually sixth through eighth grades (ages 11–14); sometimes not including sixth; sometimes fifth or ninth grades as well (never both). Schools that are specifically junior highs usually only include two grades (seventh and eighth grades), which might explain the current/recent shifting in the US away from the junior high model to the longer-running middle school model. In the Asian countries, this is usually seventh through ninth grades. A similar intermediate stage between primary school and secondary school existed in England and Wales but has been mostly discontinued. In Japan, compulsory education stops after junior high, and High School isn't state funded; expect students in the last year of junior high to be fretting over getting into the right high school, and maybe over how they will pay for it.
2. Similar to High School, except that your body embarrasses you in different ways at different times.
3. Place where children are sent to become teenagers. If walls could talk, these wouldn't. They'd just stand there and angst.
There are real differences in educational philosophy behind the name shift from "junior high" to "middle school" starting in the 1970s, as well as a matter of teacher prestige. However, due to the localized nature of American public education, some districts took these on board more or less than others, regardless of whether they actually changed the name on the building or not. Thus, in the public mind (and therefore in fiction), the two terms are used interchangeably.
Note that even though "middle school" happens to be a direct translation of the Japanese Chuugakkou, the name shift has little to do with the institutional Japanophilia of The Eighties.
Most High School tropes apply here.
In Live Action TV, expect any Middle School-set show that gets a third or fourth season to simply Retcon the existing institution into a High School, as that's much cheaper than building new sets and casting new teachers.