A major focus in the STEM public engagement sector concerns engaging with young people, typically through schools. The aims of these interventions are often to positively affect students' aspirations towards continuing STEM education and ultimately into STEM-related careers. However, most schools engagement remains in the form of short one-off interventions, which obviously have limited scope for real lasting impact. In this paper we discuss various different programmes of repeated interventions with young people, assessing what impacts can realistically be expected. Short series of interventions, rather than one-offs, appear to also suffer some limitations in the types of impacts achievable. However, deeper programmes that interact with both young people and those that influence them seem to be more effective. We discuss how developing a Theory of Change and considering young people's wider learning ecologies are required in enabling such lasting impacts. Finally, we raise several sector-wide challenges to implementing and evaluating these emerging approaches.