We explore the ways in which preservice teachers (PSTs) develop a practice "in" practice (Darling-Hammond, 2010) with diverse learners when placed in classrooms with college writing mentor teachers. Analyzing survey data and instances of stated confidence in PSTs' activity logs, we share results that reveal a significant increase in the novice teachers' perceived ability to teach diverse learners when placed in this context. Results also demonstrate a model of Teacherly Reflective Inquiry Practice (TRIP). These results suggest that placements with college writing instructors acting as mentor teachers can facilitate the development of collective efforts to teach diverse learners.