Teachers need to judge students accurately to ensure social justice within classrooms. Currently, many reviews have estimated how accurately teachers overall judge students, but only a few provided clues about how teachers' accuracy could be improved. To provide insight regarding the sources of teachers' (in)accuracy, we review and synthesise lens model studies concerning the judgement accuracy of teachers. After describing the lens model and the limitations of previous reviews, an individual participant data meta-analysis of lens model studies was performed. We describe how our approach can be utilised to enhance the accuracy of teachers' judgements by distinguishing sources of inaccuracy, particularly among teachers in need of further training, and judgement tasks that need the attention of subject matter experts.