This paper reports a case study of four project schools in the primary school context of Hong Kong. It has investigated how teachers who were assigned subject panel head's and curriculum leader's roles perceived the student assessment reform and how they experimented with new assessment strategies in the school policy and classroom practice. The major focus of the paper lies on how these teacher leaders expressed their understanding of student assessment reform in classroom practice. The main discussion of findings shows the perception of these teacher leaders on student assessment reform, how they put their ideas of formative assessment into practice, and in addition, the challenges they faced in the reform process.