This paper explores the knowledge conceptions in teachers' curriculum making within a classroom perspective through a lens of social realist theory. Curriculum making is conceptualised as a process that occurs between students, teachers, knowledge content and contextual factors, in which teachers must balance various priorities and knowledge boundaries. The analytical distinctions between knowledge conceptions, boundaries and structures are based on Young and Muller's three future curriculum scenarios and applied to empirical data retrieved from two eighth-grade (14-15 year olds) classrooms in Sweden regarding lessons on Swedish and the natural sciences. The findings show that the curriculum knowledge requirements impede the agency of teachers and shape the conditions for curriculum making. Teachers seek to balance priorities in terms of the aim and content of subject knowledge, teaching activities and their knowledge of the students. Conceptions of knowledge and boundaries are constructed in complex ways as teachers try to build bridges and engage in disciplinary boundary crossing between subject-specific knowledge and knowledge from the world of students' subjective experience. These movements and epistemic transactions in curriculum making are ways of integrating different knowledge structures, and they have the potential to encourage exploration and moving beyond social contexts and everyday experience.