Newly accessible data from the Scottish Mental Survey 1947 are used to investigate the legacy in the 1950s of reforms to Scottish secondary schooling in the first four decades of the 20th century. These reforms had sought to extend opportunity for post-primary education beyond the children of the professional middle class who had formed the largest part of the clientele of older secondary schools. The reforms also provided unprecedented opportunities to girls. The findings of the analysis are that the widening of opportunity was maintained in the stable, selective system of the 1950s. A broadly homogeneous system of academic secondary education had been created for about 45% of the age group, by means of selection at age 12. Social class continued to influence pupils' progress in this secondary sector, but not to any different extent in the older or the newer schools. Progress in secondary school was not related to gender except in science. (Contains 14 notes and 7 tables.)