A growing quantitative literature finds evidence that student teaching placements predict later outcomes of teacher candidates and their students, but there is little large-scale quantitative evidence about the mechanisms for these estimated relationships. We used data from a survey of STEM teachers in Washington State to better understand how their perceptions of preparation were related to student teaching placements and current classroom environment. We found evidence that the composition of students in student teaching classrooms was predictive of STEM teachers' perceptions of their preparation. For example, STEM teachers who student taught in classrooms with more English language learners and economically disadvantaged students reported feeling prepared to teach these specific student populations. Likewise, STEM teachers who student taught in high-poverty classrooms tended to report feeling better prepared to manage their current classroom, particularly if they were currently teaching in a high-poverty classroom.