Comprehensive schools provide an entitlement curriculum to all children, without selection whether due to financial considerations or attainment. A consequence of that is a wider ranging curriculum, including practical subjects such as design and technology and vocational learning, which were less common or non-existent in grammar schools. Providing post-16 education cost-effectively becomes more challenging for smaller comprehensive schools, because of the number of courses needed to cover a broader curriculum with comparatively fewer students. This is why schools have tended to get larger and also why many local authorities have organised secondary education into 11–16 schools, with the post-16 provision provided by sixth form colleges and further education colleges. Comprehensive schools do not select their intake on the basis of academic achievement or aptitude, but there are demographic reasons why the attainment profiles of different schools vary considerably. In addition, government initiatives such as the City Technology Colleges and Specialist schools programmes have made the comprehensive ideal less certain.
In these schools children could be selected on the basis of curriculum aptitude related to the school's specialism even though the schools do take quotas from each quartile of the attainment range to ensure they were not selective by attainment. A problem with this is whether the quotas should be taken from a normal distribution or from the specific distribution of attainment in the immediate catchment area. In the selective school system, which survives in several parts of the United Kingdom, admission is dependent on selection criteria, most commonly a cognitive test or tests. Although comprehensive schools were introduced to England and Wales in 1965, there are 164 selective grammar schools that are still in operation. (though this is a small number compared to approximately 3500 state secondary schools in England). Most comprehensives are secondary schools for children between the ages of 11 to 16, but in a few areas there are comprehensive middle schools, and in some places the secondary level is divided into two, for students aged 11 to 14 and those aged 14 to 18, roughly corresponding to the US middle school (or junior high school) and high school, respectively. With the advent of key stages in the National Curriculum some local authorities reverted from the Middle School system to 11–16 and 11–18 schools so that the transition between schools corresponds to the end of one key stage and the start of another.
In principle, comprehensive schools were conceived as "neighbourhood" schools for all students in a specified catchment area.
The first comprehensives were set up after the Second World War. In 1946, for example, Walworth School was one of five 'experimental' comprehensive schools set up by the London County Council Another early comprehensive school was Holyhead County School in Anglesey in 1949. Coventry opened two Comprehensive School in 1954 by combining Grammar Schools and Secondary Modern Schools. These were Caludon Castle and Woodlands. Another early example was Tividale Comprehensive School in Tipton. The first, purpose-built comprehensive in the North of England was Colne Valley High School near Huddersfield in 1956.
The largest expansion of comprehensive schools resulted from a policy decision taken in 1965 by Anthony Crosland, Secretary of State for Education in the 1964–1970 Labour government. The policy decision was implemented by Circular 10/65, an instruction to local education authorities to plan for conversion. Students sat the 11+ examination in their last year of primary education and were sent to one of a secondary modern, secondary technical or grammar school depending on their perceived ability. Secondary technical schools were never widely implemented and for 20 years there was a virtual bipartite system which saw fierce competition for the available grammar school places, which varied between 15% and 25% of total secondary places, depending on location.