In the education systems of England, Wales, and Northern Ireland, and of Commonwealth West Indian countries such as Barbados, Trinidad and Tobago, Belize, Jamaica and Malta, sixth form (sometimes referred to as Key Stage 5) represents the final two years of secondary education, where students (typically between sixteen and eighteen years of age) prepare for their A-level (or equivalent) examinations.

The first five years of English secondary schooling were previously known as forms. Pupils started their first year of secondary school in the first form or first year, and this was the year in which pupils would normally become 12 years of age. Pupils would move up a form each year before entering the fifth form in the year in which they would have their sixteenth birthday. 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 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.