This article presents a description of a South African mathematics teacher's struggles to mediate a new national assessment tool called the Common Tasks for Assessment with her grade 9 learners, who are unprepared for the assessment. The purpose of the study is to describe and to identify the constraints on the teacher's mediation attempts. The study is set in the context of an under-resourced classroom with a large number of learners, from disadvantaged backgrounds, who have serious problems with fundamental mathematical concepts. The analytical framework for the study draws upon actions, processes, objects and schemas theory and Valsiner's zone theory. Data for the case study were generated from seven lesson observations, two interviews with the teacher and a focus group interview with a group of learners from the class. The findings reveal that the teacher's practice was impeded by the learners' (non)readiness for the task, the task demands, the restrictions of the curriculum demands, as well as the fact that the modern assessment policy was irreconcilable with the classroom reality. The article concludes by calling for a closer alignment of curriculum implementation plans with the classroom realities of teachers such as the one presented in the study.