Central to the assembling of the New South Wales public education system in the late nineteenth and early twentieth centuries was the setting of borders and boundaries between different categories of students. These boundaries were particularly decisive in the institution of the public high school, where entry and progress were regulated by tests and examinations. Students were ostensibly grouped according to intelligence, application and age rather than older forms of categorization such as family and social class. Gender as a category was mediated by merit. Although it was known that the spread of public high schools in the United States and Canada had been facilitated by the early adoption of coeducation, the New South Wales government was initially reluctant to establish mixed-sex high schools. Its first public high schools, founded under the Public Instruction Act of 1880, were all single-sex institutions. By the early twentieth century, however, the Department of Public Instruction's policy had changed and coeducation was a significant feature of the expansion of secondary schooling in New South Wales in 1911-1912 and the establishment of a meritocratic ladder of educational opportunity. While the Department's preference continued to be for single-sex schooling until the middle of the twentieth century, its willingness to establish mixed high schools in sparsely populated places opened the way for the successful expansion of a public high school network into regional and rural areas, a movement that had been frustrated in the 1880s. There was little public debate about coeducation in late nineteenth or early twentieth-century New South Wales--unlike in England or the United States for the same period. For the most part New South Wales Education Department attitudes to and policies on coeducation--and gender--need to be inferred by reading more general kinds of discussions concerning the role of the state in higher education and the aims and purposes of secondary schooling. Nevertheless it is possible to assemble a picture of the ideas and beliefs regarding gender and high schools that informed the Department of Public Instruction's policy first to proscribe, on idealistic grounds, and then tolerate, on pragmatic grounds, its adoption in regional high schools. This article examines this policy transition by reading representative or indicative discussions from key moments in New South Wales public education history. These are: the New South Wales Legislative Council debates over the clauses to establish high schools in the Public Instruction Bill of 1880, public statements demanding educational reform at the beginning of the twentieth century, and the policies establishing the major public education settlement of 1911-1912. (Contains 53 footnotes.)