This article explores how preschool teachers, over time, collectively manage teaching as a new part of their mission. The study's empirical data consist of two related but temporally separated sets of data containing collegial discussions among preschool teachers; talks during a development process and group interviews with the same preschool teachers six months later. Through a theory-driven analysis, using the theoretical concept of teacher agency, different ways of achieving teacher agency are brought into light. When tensions appear, the preschool teachers achieve teacher agency by using professional core values in order to make adjustments, additions or changes to school policy. These professional core values consist of for example, sensitivity to the interests and needs of children, the ambition to perform a pedagogical practice for the greater good of children and the professional tradition of preschool teachers and child minders working together in teams.