Within the Swedish special school for children with intellectual disability (ID) issues concerning the improvement and formative assessment of knowledge appropriation are largely unexplored. The teaching practice has generally been organized by means of simple, repetitive tasks with a minimum of variation and a tempo that tends to inhibit, fragmentize and simplify the whole. Thereby contemplated knowledge remains inaccessible and to a simpler assessment practice. However, new demands from the government make teachers face requirements to create more complex assessment practices. This presentation is based upon data from seven case studies focusing teachers experiences from teaching and assessing literacy. The case studies were designed as research-circles, focus groups interviews, or as research and development projects. 120 teachers from different special schools together with researchers were involved. Theoretically, the projects are framed within the Cultural-Historical Activity Theory and focus on two main issues: What is the object of teachers´ assessment work concerning literacy? How do teachers handle a changed teaching assignment – from a simpler to a more complex practice? Results indicate that teachers seem to be hindered by their taken-for-granted assumptions about assessing, i.e. to shift perspective from focusing on what simply could be assessed to what would be worth assessing.  By challenging these assumptions, potential changes in assessment practices could be discussed.