This paper presents a multi-year study on the impact of attending an engineering summer school on girls' confidence that they could become an engineer. While it is relatively easy to recognise and address tangible barriers to entering an engineering profession -- such as a lack of preparedness in terms of a mathematics or physics background -- other barriers to young women undertaking engineering studies are more subtle. This paper examines whether young women have the confidence and knowledge of what is required to become an engineer, and whether it is possible to improve these factors by means of an engineering summer school for high school girls. Results show a positive impact on both the confidence of the participants and their knowledge of what tertiary engineering studies entails.