United States

In the United States, authority to regulate education resides constitutionally with the individual states. The direct authority of the U.S. Congress and the federal Department of Education is essentially limited to regulation and enforcement of federal constitutional rights. Great indirect authority is exercised through federal funding of national programs and block grants; but there is no obligation upon any state to accept these funds, and the US government otherwise may propose but not enforce national goals, objectives and standards, which generally lie beyond its jurisdiction.
Nevertheless, education has had a relatively consistent evolution throughout the United States. All states have historically made a distinction between two genres of K-12 education and three genres of K-12 school. The genres of education are primary and secondary; and the genres of school are elementary school, middle or junior high school, and high school (historically, "senior" high school to distinguish it from the junior school).
Primary education (or "primary school" meaning "primary education") still tends to focus on basic academic learning and socialization skills, introducing children to the broad range of knowledge, skill and behavioral adjustment they need to succeed in life - and, particularly, in secondary school. Secondary education or secondary school has always focused on preparing adolescents for higher education or/and for careers in industries, trades or professions that do not require an academic degree.
The elementary school has always been the main point of delivery for primary education; and the (senior) high school has always been the focal point of secondary education. Originally, elementary school was synonymous with primary education, taking children from Kindergarten through grade 8; and secondary school was entirely coextensive with the high school grades 9 - 12. This system was the norm in America until the years following World War I, because most children in most parts of what was then the mostly rural United States could go no further than Grade 8. Even when the high schools were available, they were often not accessible.
As the population grew and became increasingly urban and suburban instead of rural, the one-room schoolhouse gave way to the multi-room schoolhouse, which became multiple schools. This produced the third genre of school - the junior high school - which was designed to provide transitional preparation from primary school to secondary school, thus serving as a bridge between the elementary school and the high school. Elementary schools typically operated grades Kindergarten through 6; the junior high school, often housed in the same building as the senior high school, then covered grades 7 through 9; and the senior high school operated grades 10 through 12.
It was typical during this period for state departments of education to certify (in California, "credential") teachers to work in either primary or secondary education. A Primary School Certificate qualified the holder to teach any subject in grades K through 8, and his/her major and minor subjects in grade 9. A Secondary School Certificate qualified the holder to teach any subject in grades 7 and 8, and his/her major and minor subjects in grades 9 through 12. Certain subjects, such as music, art, physical, and special education were or could be conferred as K through 12 Teaching Certificates.
By the late 1960s, the lines of transition between primary and secondary education began to blur, and the junior high school started to get replaced by the middle school. This change typically saw reassignment of grade 9 to the (senior) high school, with grade 6 reassigned to the middle school with grades 7 and 8. Subsequent decades in many states have also seen the realignment of teacher certification, with grade 6 frequently now included on the secondary teaching certificate. Thus, whereas 20th century American education began with the elementary school finishing at grade 8, the 21st century begins with the American elementary school finishing at grade 5 in many jurisdictions.
Nevertheless, the older systems do persist in many jurisdictions. While they are in the minority today, there are still school districts which, instead of adopting the "middle school", still distinguish between junior and senior high schools. Thus, high schools can be either 9-12, which is most common, or 10-12.