This paper examines the policing of harmful sexual behaviour (HSB) among young people in schools, drawing on qualitative research conducted with police and schools in southeast England. Utilising a Foucauldian surveillance perspective, we explore the challenges police experience in balancing punitive measures with relationship-building efforts. We highlight contradictions between policing objectives and strategies, with police engagement often emphasising surveillance, intelligence gathering and detection, including among officers endorsing relationship-based practice with young people. The overarching concern with behaviour management and discipline of young people in schools, combined with inadequate training and resourcing, perpetuates authoritarian policing practices, with implications for police–youth relations. We identify how tensions between deterrence and trust play out through a wider crisis of legitimacy regarding the capacity for legal frameworks and criminal justice to adequately capture and respond to HSB. We suggest these limitations undermine young people's rights, erode trust between young people and police, and ultimately compromise safety through hindering the effectiveness of HSB prevention and response efforts.