Tackling the Science, Technology, Engineering, and Mathematics (STEM) skills shortage is an ongoing challenge for governments and policymakers worldwide. While the last decade has seen an increase in student enrolment into STEM degree courses in the UK, the gender gap remains a persistent issue in academia. In order to understand the causes underpinning female under-representation in STEM, it is essential to consider the whole academic pipeline and identify its "leaky points". As the transition between compulsory and voluntary schooling represents a critical attrition point in the STEM educational pipeline, we conducted a survey to investigate the attitude of a cohort of year 11 and year 12 pupils towards STEM disciplines. Our results indicate that a gender bias is already evident at General Certificate of Secondary Education (GCSE) level, where only 25% of girls expressed the intention to pursue a career in STEM. In both age groups, girls expressed a considerably lower preference for chemistry, physics and ICT, and a higher preference for biology compared to boys in the same year group. Participation in extracurricular activities and parents' educational status both showed a strong correlation with pupils' intention to undertake STEM studies in higher education and pursue a career in a related field. A recurring theme emerging from the survey is that pupils are eager to gain more knowledge about "real life" applications of science and are inspired by interactions with visiting scientists. These results corroborate the notion that outreach activities have a crucial role in promoting engagement in STEM subjects amongst young learners, and are essential towards the endeavour of "mending the pipeline" at an early stage.