<p>Ability grouping remains a widerspread and damaging practice in primary school mathematics classrooms, perpetuating deficit narratives, and replicating societal inequities. Despite increasing evidence supporting alternative approaches, some teachers encounter significant challenges in enacting these, as pedagogical decisions are shaped by deep-seated beliefs about fixed mathematical ability. This paper presents an in-depth case study of one primary school teacher, Meliame, who participated in a design-based collaborative intervention involving iterative cycles of out-of-class professional learning and in-class coaching support. Findings show that Meliame’s initial grouping approach generated rigid ability hierarchies rooted in fixed perceptions of capability. Through sustained, relational support, she was guided to identity individual student strengths, disrupting deficit narratives, and redesigning grouping practices accordingly. A pivotal classroom episode illustrated how deliberately drawing on a low-status student’s strength in visual art provided equitable access to mathematical reasoning and elevated his standing within the group and class. Three key factors supported successful implementation. High relational trust between teacher and researcher, explicit attention to status and expectations, and a strengths-based planning record. This study contributes an empirically grounded model for integrating strengths-based grouping into mathematics teaching, demonstrating that continuous, embedded professional learning is essential for disrupting entrenched inequities and positioning students as capable mathematical learners.</p>