This paper reports on a research project undertaken with a group (n = 19) of Irish preservice student teachers (PSTs) during the third year of a five-year undergraduate education course. A series of workshops were carried out on the classification and design of mathematics tasks. The research is presented as a case study using mixed methods to gather data at three points, before the workshops, after the workshops, and two years later, when PSTs were in their final year. We argue that task classification and design are curriculum-making activities for mathematics teachers. Through critically classifying and designing mathematics tasks, PSTs developed skills of evaluating the cognitive demand of tasks and showed evidence of using these skills in curriculum making. Through using frameworks to classify tasks, the PSTs demonstrated a capacity to take a more critical role in curriculum making. Our data show that the development of skills in these areas has lasting effects on PSTs' practices and willingness to engage meaningfully with the curriculum in mathematics. The research highlights the need for PSTs to work on evaluating and designing tasks. How frameworks can support and enhance PSTs' capacity to make curricular decisions is evidenced.