This article details key findings from a longitudinal study conducted in collaboration with Kids Company, funded by the Arts and Humanities Research Council (AHRC). The focus of this article is the role of the "teacher as witness" to the impact of a participatory theatre project with vulnerable young people. This research argues that the opportunities afforded by challenging pre-emptive constructs of vulnerable youth held by a teacher, can enable changes that take place within a workshop, which can often remain temporary and confined to the space and time of a project, to transition back into the wider school environment. Drawing upon the concept of witnessing discussed by Felman, and Laub [1992. "Testimony: Crisis of Witnessing in Literature, Psychoanalysis, and History." Abingdon: Routledge], Rymaszewska [2006. "Reaching the Vulnerable Child: Therapy with Traumatized Children." London: Jessica Kingsley], Gerhardt [2004. "Why Love Matters: How Affection Shapes a Baby's Brain." East Sussex: Routledge] and Coady [2002. "Testimony: A Philosophical Study." Oxford: Oxford University Press], this article proposes that witnessing change can offer a means of sustaining impact. Insights into this proposition are offered in an example case-study analysis of a project I conducted within a Kids Company-supported primary school setting in Lambeth, south-east London.