In educational systems around the world, there is a long tradition of using psychiatric diagnoses when providing support in the special educational field. For instance, many countries have special schools and special classes for students with psychiatric diagnoses such as Attention-Deficit Hyperactivity Disorder, Autism spectrum disorder and general learning disabilities. This case study draws on data from Denmark and investigates how this organising principle is currently being challenged. The study shows that a main challenge to the widespread use of diagnoses is a shift in how the psychiatric system diagnoses students. Factors affecting the current situation are that a) an increasing proportion of students receive more than one diagnosis, and b) children with social problems are more likely to be diagnosed with a psychiatric condition as well. The outcome is that the institutional tradition of distinguishing between support to be provided for different diagnoses comes under pressure, and a paradoxical situation occurs where the growing complexity of children's psychiatric diagnoses leads to practices where more differentiated special needs are taught within the same classrooms. The situation has led the Danish school system to search for alternative ways of organising support for students. The study investigates these new emerging methods of providing support.