The article explores the notions of children's intellectual 'ab/normality' that were conceptualised in the context of emerging special educational measures at the turn of the twentieth century and the concomitant notions of child development. From the mid-nineteenth century onwards, the medical classification of 'idiocy' provided the framework for the establishment of different educational facilities, including special classes for 'feebleminded' children. The present analysis focuses on the allocation of pupils to these classes in Switzerland guided by the premise that a one-year-long trial period, during which thousands of children of the same age were observed in school, had shaped the notion of 'normal' and 'abnormal' child development. This notion in turn provided the basis for assessing intellectual ability at the individual level. In other words, no scientific metrics teachers could use to separate the 'normal' from the 'abnormal' existed when this separation process began, but rather emerged from the process itself.