This study reports the results of a research project conducted as part of high school teachers' efforts to bring about reform of science education in their school. Two classes of Grade 8 students (N = 43) taught by the same teacher were monitored in terms of students' perceptions of their learning environment, achievement levels, and conceptual understanding of research. Quantitative methods (questionnaire, tests and examinations, GPA) and qualitative methods (interviews, videotaped lessons, artifacts) were combined while the learning environment was changed to a student-centered open inquiry format. Not unexpectedly, relationships between the Autonomy and Student-Centeredness scales of the Constructivist Learning Environment Survey (CLES) and immediate (unit test) and delays post-tests (examination subsection) were detected. On the other hand, although student-student interactions in small-groups and whole-class sessions were a significant part of the learning environment, the Negotiation scale was not related to achievement. Three case studies of representative students (in terms of achievement and perceptions of the learning environment) are featured, further underscoring the importance of monitoring reform of teaching at the classroom level.