Inequity in education is pronounced when data is used to form rigid class ability groups. Rigid grouping means that students with backgrounds that disadvantage them in education can be locked into low 'ability' class groups. In contrast, informed and flexible use of data can support adaptable and heterogeneous ability grouping, which better accommodates the diverse needs and potentials of students. This approach ensures that students are not permanently categorised but can move between groups as their abilities and needs evolve. Research drawing on international data sources like the Programme for International Student Assessment (PISA) oversimplifies the range of ability grouping practices in a country like Australia. This paper reveals the findings of a study that explored how class ability grouping practices are being used in secondary schools in two Australian states. Through analysis of a survey of 143 schools and interviews with 12 educators from Queensland and Western Australia, the study explores factors influencing class grouping decisions and characterises a spectrum of practices that are enacted. The study has implications for decision making about class ability grouping, encouraging more critical consideration of the social inequity associated with inflexible class grouping and the grouping of students into 'bottom' classes.