Middle schools in England are defined in English and Welsh law as being schools in which the age range of pupils taught includes pupils who are aged below 10 years and six months, as well as those who are aged over 12. Such schools were not permitted in the state system under the legislation introduced by the Education Act of 1944. This however was changed in the Education Act 1964, which made additional arrangements to allow for schools which crossed the traditional primary-secondary threshold at age 11. Notably, these changes did not define a new type of school, but rather permitted a variation on existing schemes, while providing for regulations which allowed the Secretary of State to determine whether such schools should be treated as primary or secondary. The move, pushed forward by Alec Clegg, then Chief Education Officer of the West Riding of Yorkshire County Council, was initially part of a process to introduce comprehensive schools in secondary education. Clegg proposed a model of middle schools for students aged 9 to 13, crossing the traditional divide at age 11 as early as 1960, with the scheme fully implemented in September 1969. Earlier moves by the local authority in Leicestershire had seen the introduction of two-tier secondary schools for students aged 11 to 14, and 14 to 18 as early as the late 1950s.
The 1964 Act was followed in July 1965 by Circular 10/65 from the then Labour government requesting that local education authorities put forward plans to introduce comprehensive schools in their areas. However, the circular offered only limited support for a change to a three-tier model of middle schools, as the minister had already requested a review of the age of transfer to secondary schools as part of the Plowden enquiry into Children and their primary schools. The Plowden report published in 1967 encouraged the development of middle schools for students aged 8 to 12, developing from existing junior schools.
The law required that all schools were classified as either primary or secondary depending on the age range of students. By 1970, over 100 middle deemed secondary schools were in operation with around 30 deemed primary schools. The number of middle schools, including combined schools for children aged between 5 and 12, rose continuously over the next decade, reaching a peak of over 1400 such schools by 1983, with the primary model rapidly overtaking the secondary following the publication of the Plowden report. However, from that time onwards, the number of middle schools fell each year. The introduction of the National Curriculum with set Key Stages aligned with the old primary/secondary model further affected the numbers of schools, with a quarter of middle schools closing in the five years after its introduction. Issues of falling rolls, and queries raised about the academic progress of students in three-tier systems led to further closures.