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(known also as intermediate school or junior high school) covers a period of education that straddles primary education and secondary education, serving as a bridge between the two. The terms can be used in different ways in different countries. The concept itself dates back to 1909, with the founding of Indianola Junior High School in Columbus, Ohio.

Junior high schools were schools in the United States that contained grades 7th and 8th. The junior high schools in the United States were mostly replaced with the middle school concept with grades 6th, 7th, and 8th. The 9th grades were moved to the high schools. Many high schools were known as senior high schools when the junior high school concept was popular.

In the United States and Canada, middle school refers to a distinct form of school organization rather than a general term for the middle level of education. Advocated by groups such as the National Middle School Association, the middle school concept is a relatively new model for the middle-level grades, contrasted with the more traditional junior high concept. North American children at this level are educated either at junior high schools or at middle schools, depending on location.

Middle schools generally include grades 6 to 8 (although they can include just 7 and 8, and a few start as early as grade 4, although that is rare) while junior high schools include grades 7 and 8 or 7 through 9.

Junior high schools are designed similarly to high schools. The faculty is organized into academic departments which operate more or less independently of one another. This is meant as a hybrid, to ease the transition from elementary school to high school for students. Sometimes they are called Intermediate schools.

The middle school concept, however, involves a group of four to six teachers from different disciplines working as a team with the same group of students of the same grade level, with each teacher teaching a different subject. This format facilitates interdisciplinary units, where the entire team teaches on the same general topic from the perspective of different disciplines.

Sometimes intermediate schools go before middle school, sometimes middle school goes before junior high school, and a few times middle school goes before intermediate school. In most cases, however, the middle school (according to the middle school concept) is seen as an alternative and a replacement to the junior high and intermediate school. The middle school format has now replaced schools using the junior high format by a ratio of about ten to one in the U.S. In Canada, the junior high concept is primarily seen in Western Canada, while middle schools to US-standards are generally only seen in Ontario and parts of Atlantic Canada, where they are sometimes called senior elementary schools.