This article deepens the knowledge of practices within school cultures and their dynamics in relation to improvement work. It investigates: Which practices can be identified in school improvement among principals at upper secondary school level? How are these practices shaped? How do they promote or prevent the improvement work of the principals? Three of six identified practices challenged the planned improvement. The significance of the study is the practice perspective which visualizes and concretizes the complexity within school cultures in relation to the dynamics of improvement work; it illuminates changes and subcultures as well as external factors shaping the culture. The practice perspective used in the study avoids descriptions of general culture types and results based on experiences, which are common challenges within school culture research. The study suggests researchers conduct practice analyses to illustrate complexity within school cultures and improvement. © 2020 The Author(s).