Despite research highlighting the negative impact of ability grouping on student outcomes in mathematics, the practice seems entrenched in New Zealand schools. The Ministry of Education launched a professional development initiative with the aim that mathematics lead teachers would drive change towards teaching in mixed-ability groups using a problem-solving pedagogy. Lead teachers were seen as uniquely positioned to promote change within schools. This study examined lead teachers' changes in their own practice and that affected in their schools following the initiative. The study used a problem-based methodology. Through careful, in-depth interviews with lead teachers from six primary schools, the researchers examined lead teachers' theories-of-action and the extent to which they were able to affect change. Findings showed different levels of change in lead teachers' own and their school's practice. Only two lead teachers made considerable changes in their own practice, and only one could fully incorporate mixed-ability grouping into school-wide practices. Analyses across the six cases revealed four factors impacting lead teachers' ability to affect change: positional authority, senior leadership support, expertise in content and pedagogy, and changing teacher beliefs to bring about change. The study concludes that while the notion of lead teachers implementing change through working closely with their colleagues seems exciting and promising, the conditions for teacher leadership to be effective need to be met. Furthermore, the case of this national initiative shows that shortcuts in professional development are ineffective, especially for a matter so disputed and entrenched as ability grouping in mathematics.