Sixth form refers to the final two years of secondary school in the English, Welsh and Northern Irish education systems. These are the years now officially labelled 'years 12 and 13' (or 13 and 14 in Northern Ireland). These post-compulsory years of education are usually devoted to study leading towards AS (after the first sixth form year) and A level (or A2 level) qualifications after the second. Sixth form was traditionally followed by higher education, or , as it is also known, tertiary education; but the curriculum is more flexible now, and may lead to other careers than those requiring a university education..
Traditionally, public schools and grammar schools, the normal types of secondary education before it became compulsory in 1870, divided their pupils by year group, or 'form' (~class). Boys (for education was largely male) were placed in th 'first form' on entry, until they showed that they deserved to be raised to the second form. (The norm was for this to be in the second year of attendance at the school; but able pupils could be promoted more quickly, and dull ones could repeat the year - in some cases more than once.) These year groups were called the 1st form, 2nd form, and so on. On average boys entered the sixth form at around the age of 16, and began to prepare for entry to university, or some equivalent professional training. Some, for example, were trained to enter the army's college at Sandhurst. The two years usually spent in this were called the lower and upper sixth. Young people in these years were, and still often are, called sixth formers.
Reorganization in the 1990s led to all years being numbered consecutively from the start of primary school. The term sixth form is still widely used, alongside the modern label 'years 12 and 13', not least because of the existence of sixth form colleges. Although the post-compulsory stage involves two years of study, it is seen as a unit. The two sections are traditionally called "upper sixth" and "lower sixth" rather than a "sixth" and "seventh". These terms are no longer as common, and people often speak of the "second year of sixth form".