From the time of the Reformation in the 16th century, the Netherlands has enjoyed a high level of basic education and comparatively high literacy rates. In the 19th century efforts were made to systematise education and to secure adequate financing for schools. As the state became more deeply involved in education, a dispute arose concerning the fate of nonpublic, mainly church-related, schools. The so-called school struggle became a major political issue and was not fully settled until 1917, when a constitutional amendment guaranteed equal, tax-paid financial support for both public and nonpublic schools. Today, about one-third of the elementary and secondary schools are public, and about two-thirds are nonpublic, mainly Roman Catholic or Protestant. School attendance is compulsory for children aged 5 through 16 years. Pupils attend a primary school for six years and then enter one of several types of secondary schools, which offer training for entering a university or other advanced institution or for pursuing a vocation. Instruction is in Dutch, except in Friesland, where classes are also taught in Frisian. In the early 1990s about 1.4 million pupils attended primary schools, about 673,600 students were enrolled in general secondary schools, and 505,300 attended vocational secondary schools.