Mental health and well-being are increasingly recognised as essential components of students' lives at school. Increasingly, school leaders are introducing programs to support students' mental health and well-being across a range of domains, including the social, emotional, and academic spheres. However, the constructs of well-being are variously defined and contested in the literature and there appears to be limited consensus on how young people's well-being should be measured. Based on hedonic and eudemonic perspectives, Keyes's (2006) has suggested a consideration of subjective well-being as a measure of positive functioning in life. He has posited a mental health continuum ranging from languishing, to moderate mental health, to flourishing, which differs from the presence of mental ill-health or disorder, such as depression or anxiety. In this study we used Keyes's framework as the basis for the selection of measures of student well-being, in order to better understand the well-being of IB students in MYP. In this presentation as part of a large scale study we report the use and efficacy of three different, but complementary measures to determine students' well-being. Diener et al.'s (2009) Flourishing Scale and Keyes's (2006) Mental Health Continuum enabled two separate measures of flourishing, each using different indicators, while Liddle and Carter's (2010) Stirling Children's Well-being Scale enabled measures of languishing and poor mental health. International Baccalaureate Middle Years Program (IB MYP) students aged 11-16, attending eight South Australian schools completed the three questionnaires. Structural Equation Models in MPlus, using the data collected from 1,930 IBMYP students, showed that the majority (55%) of students indicated that they were flourishing, although a very small group of females in the upper middle years were more likely than males and younger students to be languishing. This group of females indicated that they were experiencing difficulties and emotional concerns. The implications of our findings for schools' capacities to address the well-being needs of different groups of students, such as teenage girls, are discussed.