Students in many jurisdictions can study Mathematics at different levels in their final 2 years of secondary education. The levels of Mathematics range from standard (not involving calculus), through basic calculus, to more advanced treatments of calculus and algebra. In this context, some students can elect to study Mathematics at a level below their ability. We consider the situation in New South Wales (NSW), Australia, where most Year 12 students who apply to university are awarded a percentile ranking, namely the Australian Tertiary Admission Rank (ATAR). The ATAR reflects students’ results in the final 2 years of secondary education and frequently determines what they can study at university. As the study of Mathematics is often segregated by gender, it is of interest to explore how boys’ and girls’ choices about level of Mathematics study affect their ATAR. We analyze administrative data for 46,000 senior secondary students in NSW who completed their Year 12 in 2011 and the Longitudinal Survey of Australian Youth (LSAY) for the same cohort. Using two-level regressions that control for relevant student and school characteristics, we find that, for a given level of performance in Mathematics in Year 10, girls see greater improvement than boys in Year 12 for all levels of Mathematics except the most advanced course. Girls who study basic Mathematics achieve ATAR increments as high as girls in some advanced courses. We discuss how awareness of these results may influence students’ decisions on what level of Mathematics to study in Years 11 and 12.