In the education systems of Barbados, England, Jamaica, Northern Ireland, Trinidad and Tobago, Wales, and some other Commonwealth countries, sixth form represents the final two years of secondary education, ages 16 to 18. Pupils typically prepare for A-level or equivalent examinations like the International Baccalaureate or Cambridge Pre-U. In England, Northern Ireland, and Wales, the term Key Stage 5 has the same meaning. It only refers to academic education and not to vocational education.

In some secondary schools in Barbados and Trinidad and Tobago, the sixth and seventh years, are called Lower and Upper Sixth respectively.

Sixth Form describes the two school years that are called by many schools the lower sixth (L6) and upper sixth (U6). The term survives from earlier naming conventions used in both the state-maintained and private school systems. Another well known term is Year 12 and 13, carried on from the year group system started in primary school.

In the state-maintained sector in England and Wales, pupils in the first five years of secondary schooling were divided into cohorts determined by age, known as forms (these referring historically to the long backless benches on which rows of pupils sat in the classroom). Pupils started their first year of secondary school in the first form or first year; this being the academic year in which pupils would normally be 12 years old by August 31. Pupils would move up a form each year before entering the fifth form in the academic year in which they would be 16 years old by August 31. Those who stayed on at school to study for A-levels moved up into the sixth form, which was divided into the Lower Sixth and the Upper Sixth.

In the independent schools sector, the traditional public schools did not have a consistent naming convention, except for the Sixth Form. As well as the Upper Sixth and Lower Sixth, the public schools used and still use a variety of descriptions for lower forms, such as Shell, Remove, Lower Fourth, Upper Fourth, Lower Fifth, Middle Fifth, Upper Fifth.

In some private schools, the term Middle Sixth was used in place of Upper Sixth, with the latter being used for those who stayed on for an extra term to take the entrance examinations that were previously set for candidates to Oxford or Cambridge universities. Other schools described these Oxbridge examination students as being in the Seventh Form or Third Year Sixth.

In the state sector, the system was changed for the 1990–1991 academic year and school years are now numbered consecutively from primary school onwards. Year 1 is the first year of primary school after Reception. The first year of secondary school is Year 7. The Lower Sixth (the first year of sixth form) is Year 12 and the Upper Sixth (the second year of sixth form) is Year 13. Public (fee-charging) schools, along with some state schools, tend to use the old system of numbering.

In some parts of the country, specialist sixth forms were introduced, not as part of a secondary school but rather catering solely to sixth form aged students. A large proportion of English secondary schools no longer have an integral sixth form. This is mainly related to reforms in the later 20th century, where different political areas became a factor in the introduction of colleges instead of the original sixth forms. There are now numerous sixth form colleges throughout England and Wales, and in areas without these, sixth form schools and specialist further education (FE) colleges called tertiary colleges may fill the same role. As of 2015^[update], there were 93 sixth-form colleges in England.