In 1940, there were 156 schools in New Zealand offering secondary education: 39 secondary schools, 96 district high schools, and 21 technical high schools.
In 1944, the school leaving age was raised from 14 to 15. At the same time, a gradual move started away from separate secondary schools and technical high schools toward comprehensive secondary schools serving both, and district high schools started falling out of favour to separate secondary schools. Combined with the post-World War II baby boom, the number of secondary students swelled and a large number of new secondary schools had to be built. By 1960, the number of secondary students had tripled from 39,000 to 140,000, and the number of secondary schools had increased to 239, comprising 102 secondary schools, 96 district high schools, and 41 technical high schools.
With the large number of new schools being built, most state secondary schools in the 1950s, 1960s and 1970s were built to common designs. The first common design was the Naenae type (after its first use at Naenae College) in 1953, with schools built with long two-story classroom wings of mixed concrete and timber construction. This was followed by the Henderson type (after Henderson High School), which was a single-story all-timber version of the Naenae type. Starting in 1957, secondary schools moved to consisting of blocks of classrooms. The Nelson plan (named for the city in which it was designed) first started with single-storey timber buildings with 6 classrooms, before evolving into two-storey H-shaped timber buildings containing 12 classrooms in 1960. The design lasted over a decade, with the Nelson classroom block going on to become the most numerous type of classroom block found in New Zealand secondary schools. In 1971, the Nelson plan was replaced with the S68 plan (after the 1968 prototype at Porirua College), which featured single-storey classroom blocks of concrete block construction, with low pitched roofs, and internal open courtyards. In 1980, two new common design plans were introduced: the Whanau plan used at Macleans College, and the Leeston plan used at Ellesmere College. However at the same time, student numbers plateaued and secondary school construction dwindled, resulting in common design plans being dropped in 1982 in favour of schools being individually designed.
By 1980, there were 265 secondary schools and 35 district high schools, with technical high schools having been completely phased out.
In 1989, the school leaving age was raised to the present age of 16. Also in 1989, the Tomorrow's Schools reform was implemented, moving the governance of secondary schools from district education boards to individual school communities through elected boards of trustees.