<p>This study investigates how teachers form expectations of students by experimentally manipulating information about learning engagement, social group membership, and instructional context in a factorial survey of 403 Danish primary school teachers from 45 schools of 12/98 Danish municipalities. This study extends existing research by linking teachers’ assessments of student engagement to their expectations of academic achievement while controlling for social group characteristics, thereby identifying more actionable targets for interventions against teacher expectation bias. The experimental results identified four main findings. First, teachers anchor their expectations of students’ achievement potential in observable classroom behaviours rather than social group characteristics. Second, expectation bias against students from families with low parental involvement persists, even when teachers have the motivation and opportunity to prioritise richer, contextually relevant information about engaged behaviours. Third, instructional context shape how engagement translates into expectations of higher achievement potential. Teachers associate engagement in student-centred activities with higher potential than engagement in teacher-directed activities. Fourth, teachers prioritise surface-level behavioural engagement – visible participation or on-task performance – over less visible forms, such as mental effort and self-regulation. These findings support a contextual theory of action in teacher expectation research – conceptualising expectation formation primarily as an emergent property of teacher-student interactions. Teachers’ expectations appear primarily shaped by their assessments of student engagement within contextual and cognitive constraints, underscoring the need for interventions that align our understanding of teacher expectation formation with real classroom conditions.</p>