Although education is not compulsory in Singapore, primary school is free for six years, and attendance is nearly universal. Some 73 percent of children also attend secondary school. Since 1987 English has been the language of instruction, but a policy of bilingualism requires that children also be taught Chinese, Malay, or Tamil. Institutions of higher education include the National University of Singapore and Nanyang Technological University. Of Singaporeans aged 15 and older, 100 percent can read and write.