Classroom learning environments are frequently assumed to exert their influence on learning indirectly, via student engagement. The present study examined the influence of environmental challenge and support on learning in high school classrooms, and the potential for student engagement to act as a mediator in this relationship. Data were collected in seven classrooms in six different subjects in several US high schools. The 104 students in these classes participated in the Experience Sampling Method (ESM) and reported records (N = 254) of engagement, learning, and related experiential variables. Measures of the learning environment were also rated from video footage. Variations in the learning environment observed and rated from video were linked to students' real-time reactions to instruction synchronously. Results indicated that environmental support, but not environmental challenge, was significantly related with perceived learning. Multi-level path analyses revealed that the association between environmental supports and learning was mediated by student engagement. This mediating relationship held specifically for two components of environmental support: Motivational supports and supportive relationships. Implications are discussed for the benefit of practicing school psychologists, including strategies for facilitating motivational and relational support to enhance student engagement.