This study of institutional categorisation analyses how municipalities, psychiatric clinics and schools conceive of and act on school difficulties in the Danish school system. The research analyses the responses of municipalities and schools following changes in institutional practices for the allocation of special educational support. The main data consist of documents and of interviews with professionals involved in processing students' cases and making decisions regarding provision of special educational support. The main results show that ways of understanding and handling school difficulties at the different levels of decision-making change when incentives to use psychiatric diagnoses (ADHD and other) are reduced. When school problems occur in the system launched during recent years, institutional actors increasingly look to the school context to locate the core of the problem and to search for its solutions in organisational structures and pedagogical practices. The consequences of these shifts in institutional practices and accountabilities are discussed.