Part-time engagement is a significant feature of senior secondary study for large numbers of students, in a context which includes high level policy pressures to achieve better outcomes for many more and a wider proportion of the schooling population, and significant policy and practice shifts at school, system and accreditation authority levels. Yet little is currently known about which students choose this option, and why, or the policy, schooling practice, system or theoretical implications of this change in senior secondary engagement patterns. The paper argues that research is required to fill this gap by identifying the intersections between part-time study and retention, engagement and completion, within broader theoretical, contextual and educational frameworks around post-compulsory study, and youth transitions more generally. It highlights definitional and statistical complexities which confuse understandings and distort analyses of changing patterns of senior secondary engagement, and the disjuncture between these definitions and the lived realities of students and their schools. Rather than trying to capture this shifting and fluid educational engagement through definitional gymnastics and statistical manipulation, the paper concludes by suggesting the concept of extended completion as a more useful descriptive and analytical tool. And it introduces a research project designed to fill the research and policy gap explored below, and to enhance retention and completion policy and practice. (Contains 3 tables, 1 figure, and 1 note.)