This thesis is premised on “a politics of becoming” (Gowlett, 2013, p. 149), a Deleuzo-Guattarian notion which speaks to social justice research. Rather than a focus on reductionist reformist politics, I explore moments of possibility as lines of flight that disrupt dominant discourses. As outlined in the New Zealand Curriculum, New Zealand schools are charged with the task of strengthening students’ key competencies (Ministry of Education, 2007a) to lay a foundation for lifelong learning. Learner agency is embedded in a dispositional view of these competencies but there is a paucity of research from a poststructural perspective in this area from New Zealand. Agency is also fundamental to a sociocultural conception of assessment for learning (AfL) where learners initiate, participate and contribute to learning in their classroom communities. Positioned in theoretical landscapes of socioculturalism and feminist poststructuralism, this study investigates agency through a rhizo-textual analysis in two year nine classrooms. The dynamic poststructural view of agency theorised in this thesis is derived from Judith Butler’s (1993) notion of performativity which precludes any prediscursive autonomous subject. Using data from episodes in two year nine classrooms I explore: how students engage as authoritative, active participants, authoring and directing their own actions in social activity within multiple discourses; how students move themselves from one set of culturally and socially structured subjectivities to another; and how agency can look, sound and feel in the discursive space of the classroom. In keeping with a rhizoanalytic approach, I construct plateaus of discourse based on episodes of classroom activity. These three short episodes of classroom discourse serve to illuminate the subjectivities in play. There are two forms of analysis used to construct these plateaus. Firstly, I conduct a discourse analysis of identity affordances and discourses to examine the nature of learner positioning. I then use rhizo-textual analysis (Honan & Sellers, 2006) to map the students’ and teachers’ moves in discourse and shifting subjectivities. The findings highlight how agency can appear as a rapid series of rhizomatic discourse moves that take place as students and teachers deterritorialize and reterritorialize discourses as they enact specific identities. They resonate with Davies’ (2000) observation that learners can accept, resist, subvert and change or ignore a range of discourse positions. The study also illustrates that what can appear to be ‘off-task’ behaviour can be also read as highly agentic. The dynamic and rhizomatic theory of agency proposed illustrates that learners can inhabit multiple subject positions across discourses as they respond to the interpellations of their teachers and peers. Rather than a performance where individuals act out roles as pre-discursive identities, students exercise performativity within and across classroom discourses as they are constituted agentically through their lines of flight. The research makes a methodological contribution through combining sociocultural and poststructural theories to explore the discursively constructed social and cultural environments of two classrooms. This is a deterritorializing move away from conventional sociocultural learning theory to incorporate an ecological (Boylan, 2010), rhizomatic view of classroom participation. This research has implications for how educators conceptualise learners’ identities and provide affordances for learners to initiate learning and take up agentic positions in classroom discourse. It also has implications for the ways in which the key competencies can be interpreted and strengthened in classrooms. Rather than ‘having’ agency to transfer competencies from one situation to the next, competencies are produced and enacted as learners shift subjectivities within and across discourses. The findings also offer students, teachers and policy makers insight into the learning dynamics of classrooms which embody the ‘spirit’ of AfL (Marshall & Drummond, 2006) where students can be afforded opportunities for lines of flight to initiate learning. Through being aware of learners’ rhizomatic moves, teachers may be able to notice, recognise and respond to learner initiatives more readily, and assist them to develop their capacity to be agentic learners.