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), and sometimes ninth grade, which might explain the current/recent shifting in the US away from the junior high model to the longer-running middle school model although ninth grade being part of it was much more common in the 1950s-70s. 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.
In Israel, which used to have first-through-eighth grades in elementary and ninth-through-twelfth high schools, seventh-through-ninth middle schools were introduced to reduce the shock from the stark changes from elementary to high school, which caused many students to drop out. This backfired when it turned out they were actually increasing disparities between students, and nowadays many schools are middle-cum-high schools.
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. This is common enough to be a trope in itself: Middle School Is Miserable.
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. The older term's stickiness in media is not only due to Most Writers Are Adults but that Los Angeles Unified School District was a late adopter, only changing over in the 1990s.
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 '80s.
Most High School tropes apply here.
Junior High is a common setting in Middle Grade Literature. 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.
