Practicum is a highly valued component of teacher education programs, providing opportunities for preservice teachers to observe, learn and trial effective teaching in partnership with experienced mentor-teachers. To date, little research has investigated how school-based practicum relationships are interactively achieved, turn-by-turn, in situ. Employing a single case study, this paper analyses a conversation between a mentor teacher and a preservice teacher about high-stakes assessment of the teacher-mentee's practices. Methods derived from conversation and membership categorisation analysis reveal that the participants assemble a pleasant but asymmetrical mentoring relationship by means of a continuing three-part conversational structural arrangement in this particular educational site, talking-into-being mutually acceptable versions of teachers and of teacherly practices. However, opportunities for mutually mediated learning and relational knowing were lost. These findings suggest the need to examine additional examples of similar school-based mentoring conversations about evaluation to see whether the analytic phenomena revealed here are typical or atypical of such institutional interactions.