This study investigated whether and how students with low prior achievement can carry out and benefit from reflective assessment supported by the Knowledge Connections Analyzer (KCA) to collaboratively improve their knowledge-building discourse. Participants were a class of 20 Grade 11 students with low achievement taking visual art from an experienced teacher. We used multiple methods to analyze the students' online discourse at several levels of granularity. Results indicated that students with low achievement were able to take responsibility for advancing collective knowledge, as they generated theories and questions, built on each others' ideas, and synthesized and rose above their community's ideas. Analysis of qualitative data such as the KCA prompt sheets, student interviews and classroom observations indicated that students were capable of carrying out reflective assessment using the KCA in a knowledge building environment, and that the use of reflective assessment may have helped students to focus on goals of knowledge building. Implications for how students with low achievement collaboratively improve their knowledge-building discourse facilitated by reflective assessment are discussed.