Australia currently faces a skill shortage in Science, Technology, Engineering and Mathematics (STEM) professions unlikely to improve if the current trend of low enrolments in high-level mathematics in secondary school continues. Many factors seem to contribute to this trend, and amongst them, research recognises student "attitudes" towards mathematics and careers in STEM. Research also shows that teachers and classroom practices are key agents to change these attitudes. This paper examines teachers' perceptions of students' attitudes towards mathematics and careers in STEM, as well as classroom practices emanating from those perceptions. Combining data from two studies, a large-scale survey and an intervention, we argue three main points: (1) student attitudes and teachers' perceptions of those attitudes quite clearly differ, (2) targeted interventions showing links between mathematics and careers in STEM can have an influence on students' perceptions of STEM careers and (3) an implementation of classroom ability grouping based on standardised test scores can be disadvantageous to students who have a natural inclination towards STEM and positive attitudes towards mathematics.