On becoming independent in 1956, Tunisia inherited a small but efficient educational system based on French and, to a lesser extent, Islamic influence. In 1958, the government nationalized most of the existing facilities; remaining private institutions were subject to government regulation. In the same year the government began a comprehensive plan for educational development to achieve universal, free, compulsory primary education and a significant expansion of the secondary school system. Arabic is the language of instruction in early primary grades but is later replaced by French. In 1997/1998 there were 1,440,479 students and 59,798 teachers in 4,417 primary schools. At the secondary level, there were 833,372 pupils and 36,528 teachers in general education in the same year. The pupil-teacher ratio at the primary level was 23 to 1 in 1999. In the same year, 98% of primary-school-age children were enrolled in school, while 68% of those eligible attended secondary school. The University of Tunis was founded on March 31, 1960. All higher-level institutions in 1996/1997 had 121,787 students and 6,641 instructors. Projected adult illiteracy rates for the year 2000 stand at 20.2% (males, 18.6%; females, 39.9%). As of 1999, public expenditure on education was estimated at 7.6% of GDP.