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High school or secondary school is the name used for the last segment of compulsory secondary education in Australia, Brazil, Canada, Spain, Hong Kong, India, the Republic of Ireland, Japan, the Republic of Korea, Malaysia, Mauritius, New Zealand, the Philippines, Singapore, South Africa, the People's Republic of China, the United Kingdom, and the United States. It is preceded by primary education.

The idea was first instituted in France by Napoleon as a way to train future officers for his military.

In the United States, high schools generally consists of grades 9, 10, 11, and 12, although the inclusion of grade 9 varies by school district. Students usually graduate from high school in the year of their 18th birthday if they were born between January 1 and August 31, or in the following year if they were born between September 1 and December 31, except in New York (where all students usually graduate in the year of their 18th birthday) and California (where students usually graduate in the year of their 18th birthday if they were born between January 1 and November 30, or in the following year if they were born between December 1 and December 31). A few American schools still incorporate grades 7 through 12, but the norm is usually either grades 9-12 or grades 10-12. For purposes of the GPA and subject requirements used for college admission, grade 9 is usually considered the first year of high school regardless of whether the student is in the last year of a 7-9 junior high program, or the first year of a 9-12 high school program. High school is generally defined as being grades 9-12, while "secondary" school, or "secondary" education refers to grades 7-12. Secondary Education teachers are certified to teach grades 7 through 12.

About 90% of American students complete high school. A high school diploma or GED certificate is usually required for entrance into a two or four-year college or university and to other post-secondary education programs.

As a practical matter, while laws in most states mandate school attendance at least until graduation or age 16, enforcement of the truancy laws is sporadic. Conversely, students who have failed a grade may remain in high school past the age of 18. In general, students over 18 attend alternative high schools, with the end result being attainment of a GED. State laws vary on the cut-off age for students to receive free public education services. Students in New York state can attend regular public schools until the age of 21.