This article examines what constitutes students' disruptive and good behaviour, as described and defined by teachers. Teachers are viewed as professional experts who produce official information regarding their students. The present study analysed the overarching features of behaviour descriptions provided by teachers in official statements regarding students whom they considered to be problematic, in Northern Finland between 1968 and 1991. The analysis showed that there was no common understanding of what constitutes good or bad student behaviour; behaviour assessment functioned as a ground for reinforcing power-relations and making normative comments; the school context was not addressed in the official statements; in the assessments, the focus was on bad behaviour and the form used to make the statements regulated teachers' answers. The article concludes that social problems in schools were attributed to individual students and their families through official behavioural assessments, while schools' social environments or their norms were not considered as being related to student behaviour.