Supporting productive disciplinary engagement (PDE) in low-achieving students is an important but challenging goal in education. This study used a knowledge-building inquiry approach augmented by reflective assessment to facilitate low-achieving students' PDE. A quasi-experimental design method was employed to examine the effects of reflective assessment in supporting low-achieving students' PDE. The experimental class of tenth graders (n = 20) conducted inquiries in the visual arts in a knowledge-building design augmented by reflective assessment, while the comparison class of tenth graders (n = 14) conducted inquiries in a regular knowledge-building design without reflective assessment. This study lasted approximately 4 months and the primary data source was the online discourse. A comparative analysis of the knowledge-building discourse characteristics and the sequential patterns of the discourse moves revealed higher cognitive, emotional and epistemic engagement in the experimental class compared with the comparison class. Epistemic network analysis showed that reflective assessment facilitated low-achieving students' collective reflection, monitoring and regulation, as shown by their metacognitive discourse moves. Their metacognition further helped them to achieve higher levels of cognitive, emotional and epistemic engagement than the comparison class. This study provides insights into the connections between inquiry, engagement and assessment. Moreover, it has implications for designing technology-supported collaborative inquiry environments to support low-achieving students' engagement and higher-level skills.