As schools become living sites of evidence-based practice, teachers increasingly accumulate large quantities of observations and records. In these times of an overabundance of documentation, there is a need to find the "unit of analysis" that determines the essence of what matters for assessment. In drawing upon cultural-historical theory, this paper presents the outcomes of a study which examined how 11 teachers from one primary school used the concepts of the social situation of development, motives, the zone of proximal development and the relations between the real and ideal forms of development in order to change their assessment practices. Findings show the tensions and struggles that emerged as teachers worked against the discourses associated with traditional institutionalised assessment practices where age dominates, and again as they worked with key concepts to theorise new ways of conceptualising and enacting assessment for building a new "assessment pedagogy" for their school.