Understanding how classrooms influence a student's motivation and engagement is a persistent concern in the field of educational psychology. Social-cognitive theories postulate that individual students' perceptions of their environment influence their motivation and subsequent behavior. However, empirical research directly testing the core tenets of this supposition--that changes in an individual's perceptions lead to changes in their motivation or behavior--is sparse, largely because of normative methodological approaches focusing on between-person rather than within-person comparisons. Using data from 3 time points (fall, winter, and spring) within 1 academic year in a sample of 910 middle and high school students in 68 classrooms, we modeled the bidirectional effects of within-person changes in students' perceptions of their classroom's autonomy support and academic press on their mastery goal orientation and behavioral engagement. Contrary to prior research, classroom climate perceptions were unrelated to a student's motivation and engagement. However, increases in individual students' behavioral engagement positively influenced their subsequent mastery goal orientation, and bidirectional associations between students' perceptions of autonomy support and academic press were found. This pattern of results suggests that changes in individual students' perceptions of autonomy support influence their perceptions of academic press, and vice versa, but changing perceptions are unrelated to a student's changing motivation and engagement. Results are discussed in terms of methodological considerations in testing existing theory and recent interventions to change students' motivation.