Education takes place in three stages: primary, secondary, and pre-university education. Only the primary level is compulsory. Students begin with six years of primary school, which is made up of a four-year foundation course and a two-year orientation stage. The curriculum is focused on the development of English, the mother tongue, mathematics, and science. Secondary school lasts from four to five years, and is divided between Special, Express, Normal (Academic), and Normal (Technical) streams in each school, depending on a student's ability level. The basic coursework breakdown is the same as in the primary level, although classes are much more specialised. Pre-university education takes place over two to three years at senior schools, mostly called Junior Colleges. As alternatives to Pre-U education, however, courses are offered in other post-secondary education institutions, including 5 polytechnics and the Institutes of Technical Education (ITEs). Singapore has six public universities of which the National University of Singapore and Nanyang Technological University are among the top 20 universities in the world.