This article examines high school teachers' engagement of newcomer English learner students' prior knowledge. Three central research questions guided this study: 1) To what extent do teachers function as mediators of their students' prior knowledge? 2) What goes into teachers' thinking about how and when to elicit prior knowledge? and 3) How do students respond to teachers' engagement of their prior knowledge? Data are analyzed through sociocultural perspectives on teaching, learning and identity (Lantolf, 2006; Vygotsky, 1978) and suggest that eliciting prior knowledge, particularly among vulnerable student populations, is an ethical enterprise involving risk and an ability to anticipate unexpected consequences. Implications for research and pedagogical practice are presented.