labour" for the 7th year of education. Such a system was a complete failure, however, and in 1928 the new programme completely abandoned the complex themes and resumed instruction in individual subjects. All students were required to take the same standardised classes. This continued until the 1970s when older students began being given time to take elective courses of their own choice in addition to the standard courses. Since 1918 all Soviet schools were co-educational. In 1943, urban schools were separated into boys and girls schools. In 1954 the mixed-sex education system was restored. Soviet education in 1930s–1950s was inflexible and suppressive. Research and education, in all subjects but especially in the social sciences, was dominated by Marxist-Leninist ideology and supervised by the CPSU. Such domination led to abolition of whole academic disciplines such as genetics. Scholars were purged as they were proclaimed bourgeois during that period. Most of the abolished branches were rehabilitated later in Soviet history, in the 1960s–1990s (e.g., genetics was in October 1964), although many purged scholars were rehabilitated only in post-Soviet times. In addition, many textbooks - such as history ones - were full of ideology and propaganda, and contained factually inaccurate information (see Soviet historiography). The educational system’s ideological pressure continued, but in the 1980s, the government’s more open policies influenced changes that made the system more flexible . Shortly before the Soviet Union collapsed, schools no longer had to teach subjects from the Marxist-Leninist perspective at all. Another aspect of the inflexibility was the high rate at which pupils were held back and required to repeat a year of school. In the early 1950s, typically 8–10% of pupils in elementary grades were held back a year. This was partly attributable to the pedagogical style of teachers, and partly to the fact that many of these children had disabilities that impeded their performance. In the latter 1950s, however, the Ministry of Education began to promote the creation of a wide variety of special schools (or "auxiliary schools") for children with physical or mental handicaps. Once those children were taken out of the mainstream (general) schools, and once teachers began to be held accountable for the repeat rates of their pupils, the rates fell sharply. By the mid-1960s the repeat rates in the general primary schools declined to about 2%, and by the late 1970s to less than 1%. The number of schoolchildren enrolled in