A secondary school, high school, or senior school, is an institution that provides secondary education. Some secondary schools provide both lower secondary education (ages 11 to 14) and upper secondary education (ages 14 to 18), i.e., both levels 2 and 3 of the ISCED scale, but these can also be provided in separate schools. There may be other variations in the provision: for example, children in Australia, Hong Kong, and Spain change from the primary to secondary systems a year later at the age of 12, with the ISCED's first year of lower secondary being the last year of primary provision.

In the United States, most local secondary education systems have separate middle schools and high schools. Middle schools are usually from grades 6–8 or 7–8, and high schools are typically from grades 9–12. In the United Kingdom, most state schools and privately funded schools accommodate pupils between the ages of 11 and 16 or between 11 and 18; some UK private schools, i.e. public schools, admit pupils between the ages of 13 and 18.

Secondary schools follow on from primary schools and prepare for vocational or tertiary education. In high and middle income countries, attendance is usually compulsory for students at least until age 16. The organisations, buildings, and terminology are more or less unique in each country.

In the ISCED 2014 education scale, levels 2 and 3 correspond to secondary education which are as follows:

Within the English-speaking world, there are three widely used systems to describe the age of the child. The first is the "equivalent ages"; then countries that base their education systems on the "English model" use one of two methods to identify the year group, while countries that base their systems on the "American K–12 model" refer to their year groups as "grades". The Irish model is structured similarly to the English model, but differs significantly in terms of labels. This terminology extends into the research literature. Below is a comparison of some countries:

Schools exist within a strict legal framework where they may be answerable to their government through local authorities and their stakeholders. In England there are six general types of state-funded schools running in parallel to the private sector. The state takes an interest in safeguarding issues in all schools. All state-funded schools in England are legally required to have a website where they must publish details of their governance, finance, curriculum intent and staff and pupil protection policies to comply with The School Information (England) (Amendment) Regulations 2012 and 2016. Ofsted monitors these.

School building design does not happen in isolation. The building or school campus needs to accommodate:

Each country will have a different education system and priorities. Schools need to accommodate students, staff, storage, mechanical and electrical systems, support staff, ancillary staff, and administration. The number of rooms required can be determined from the predicted roll of the school and the area needed.