I went to Catholic schools in NYC. The local public school was k-6 and the junior high was 6-9 - but they were in the same building. The public high schools were 9-12 because they had to accommodate the kids from the 1-8 schools.
The district where I live in Virginia is a bit strange, most of the elementary schools are k-5, middles school is 6-8, and 9-12 for the high school - with 2 exceptions. There are 2 islands which have their own schools even though they are part of the same district. One island town is one school k-12, the other has 2 schools, k-5 and 6-12.
When I went to school (back in the dark ages), elementary school (public because parochial schools did it differently) was k-6, junior high school was 7-9, and high school was 9-12. I know that California did things differently even then. When my child went to school in California it was elementary k-5, middle school 6-8, and high school 9-12.
In very small districts I can see k-12 or even the k-5 and 6-12 break. My grandmother taught in a one room school house where every one who wanted to learn was there. those were very different times.