Background: Within classroom research, there has long been agreement on the importance of dialogue and discussion for student learning. However, how the concept of knowledge influences classroom conversation needs further investigation, as this has key implications for students' active participation. Purpose: Our research sought to conceptualise the assumption of knowledge in standards-based curricula and explore some implications of teaching based on this kind of curriculum. To do this, we drew on a larger research project undertaken in Sweden, which involved a comparative classroom study. Methods: Four natural science lessons were examined in terms of student's opportunities to engage in the teaching content. The analytical framework was based on curriculum theory, the concepts of a lesson as a curriculum event, and students as co-authors of teaching content. We analysed two concepts of knowledge - social realism and transactional realism - in relation to an openness towards 'critical moments' during lessons, either noticed or unnoticed, and related them to the logics of curriculum and knowledge. Findings: When framed by classroom teaching designed from knowledge criteria, students' opportunities for acting as co-authors can become restricted, with critical moments overlooked because of a teaching focus necessarily limited by curriculum. Thus, opportunities for creating spaces to pay attention to students' questions and reactions can be constrained. Conclusions: Standards-based curricula, a concept of knowledge with a strong focus on subject-specific facts and ways of reasoning, together with high-stakes assessment, may lead to fewer openings for genuine discussion and student reflection. This highlights the need to leave larger spaces for teachers and students alike to influence content that engages students.