Learner agency is often seen unproblematically as an integral aspect of "twenty-first century lifelong learning". Agency and instrumentalist forms of teacher professional development are problematised through this qualitative case study that explores a teacher's inquiry into assessment for learning practices. This article illustrates how Teaching as Inquiry can be located as either a process that is purely technicist or replete with potential for teachers to engage with "unwelcome truths". Student voice data that reveal "unwelcome truths" can provide a catalyst for teacher reflection on student positioning in learning relationships. Findings focus on practices of assessment for learning, assessment literacy, agency as a neoliberal construction and the need to interrogate power relations in the classroom for student agency to flourish. Issues around student voice and the democratic participation of students in schooling improvement are discussed. Teaching as Inquiry, that explicitly targets wider non-instrumental goals associated with teacher learning, can be a critical process that addresses the growth of assessment literacy and learner agency in classrooms. It can transcend a linear obsession with easily measured quantifiable shifts in student achievement data.