Schools today face the challenge of preparing students to live, work, and prosper in a rapidly changing world. As a response to this global challenge, Norway has adopted a national curriculum focusing on the development of 21st-century skills. In this study, we investigate if and how the tasks in science and language arts textbooks in upper-secondary school have changed after the curriculum reform. We conduct a content analysis of 5,067 tasks in science and language arts textbooks and compare them to tasks in textbooks published before the reform, which we analysed in two previous studies. The results show only a marginal change in tasks in each subject, indicating that the tasks do not present the students with sufficient opportunities to practice the competences highlighted in the new curriculum. As a possible explanation regarding why textbook tasks in Norway--as well as a number of other countries--appear to change so little over time, we advance the hypothesis that the formulation of tasks in textbooks is influenced and constrained by culturally specific genre norms. These norms may represent a challenge to curriculum implementation and school change, and it is therefore important to raise awareness of textbook tasks.