Flipped classroom has been found to positively influence student achievement but the magnitude of the effect varies greatly according to discipline and local design, and few studies have been methodologically rigorous enough to establish causal evidence. Using a randomized controlled trial, this study addresses a gap in current knowledge by exploring how student responses mediate the impact of flipped classroom on academic achievement. The empirical setting is a first-year undergraduate macroeconomics course with 415 students. Comparing students in the treatment group with those in a traditional class, we find a positive, yet statistically insignificant effect on academic achievement. However, this overall effect masks important mediating effects, as students were unexpectedly reluctant to actively participate in the flipped classroom intervention. Consequently, the intervention has a substantially greater effect on academic achievement when controlling for the mediating effect of student participation which leads to consideration of the challenges of student resistance to flipped classroom.