The sixth form, in the English, Welsh and Northern Irish education systems, is the term used to refer to the final two years of secondary schooling (when students are about sixteen to eighteen years of age), during which students normally prepare for their GCE A-level examinations. The term is used to describe the final two years spent in a secondary school as opposed to a college (UK use) where students start at age sixteen after leaving secondary school.

The first five years of English secondary schooling used to be referred to as first year (in which pupils would have their twelfth birthday) to fifth year (in which they would have their sixteenth). The last two years of schooling were for the study of the old 'A' level syllabus, which are still non-compulsory, and are generally referred to as the sixth form as a relic of this older numbering system, divided into lower sixth and upper sixth, meaning years twelve and thirteen, respectively. In many private schools, the term middle sixth was used in place of upper sixth, with the latter being used as for those remaining on in the last term to take entrance examinations that were previously set for candidates to Oxford or Cambridge Universities. The modern numbering system in fact begins with Primary Education, "Year One" actually referring to the second year of schooling (the first being referred to as "Reception").

In some parts of the country, special "sixth form colleges" were introduced during the decades from 1960 onwards, recognising this as a particularly important phase of student life. A large proportion of English secondary schools no longer have an integral sixth form. This is mainly related to the liberal reforms in the later 20th century, where different political areas become a factor in the introduction of colleges instead of the original sixth forms.