The "flipped classroom" is an educational strategy about inverting the traditional use of in-class time for conducting lower-level learning activities and out-of-class time for conducting higher-level learning activities. "Guided social inquiry learning" (GSIL), which is a scaffolded constructivist pedagogic approach, has been conventionally adopted in learning and teaching of "Liberal Studies" (a "young" core senior secondary subject of social and humanities education in Hong Kong). In this research, we aimed to integrate the idea of the flipped classroom into the process of GSIL for promoting students' learning achievement and self-efficacy in studying Liberal Studies. Apart from delineating the pedagogic design of the proposed "'flipped' social inquiry learning" ("FSIL") approach, this paper also reports our quasi-experimental study on investigating the pedagogic effectiveness of FSIL in comparison with GSIL. There was a total of 215 Grade-11 students from top, middle, and bottom academic-banding schools participating in this study. Results indicated that, compared to the conventional approach, FSIL had different degrees of positive effects on the high, moderate, and low academic-achieving participants. The findings provide grounds for further studying a wider adoption of the flipped classroom in social and humanities education, as well as in constructivist learning and teaching activities.