The study was conducted in a school for special education with small classes of six to 12 students. The aim was to illuminate how students’ understanding of written assessments could be related to assessment for learning. Twenty-nine students were asked to describe their understanding of written assessment in three different subjects and, further, to describe their perceptions in follow-up interviews. The two-step analysis first focused on individual understanding. Thereafter Vygotsky’s zone of proximal development (ZPD) was used as analytical tool. In the first analysis three categories were identified. Students’ understandings were related to subject matter knowledge, generic skills and attitudes. The findings of the second analysis indicated that written assessment often lacked formative features and cannot be seen as learning in the ZPD. Difficulties with schoolwork were seen as problems of the individual and the measures were directed towards the student. Even though the guidelines ask for a relational perspective a categorical perspective sometimes was adopted in the assessments. The implications of the study relate to the ways teacher’s assessment competences can encompass students’ understanding and how this can be translated into action.