This study designs and evaluates the effectiveness of a teaching process for the topic "Density and Fluid Pressure" (8th-grade Natural Science) aimed at developing critical thinking skills for junior high school students. The pedagogical process integrates the Science - Technology - Society - Environment (STSE) model with the Flipped Classroom approach, contextualized through the problem-based scenario "Ocean Cleanup Campaign". A quasi-experimental design was conducted on 100 8th-grade students at Dong Xuan Junior High School (Phu Tho Province), divided into an experimental group (Class 8A1, n=50, implementing the STSE flipped process) and a control group (Class 8A2, n=50, taught via traditional instruction). Assessment instruments included individual pre- and post-tests evaluated using a 4-criterion Rubric (10-point scale, inter-rater reliability ICC = 0.88) combined with qualitative analysis of group discussion transcripts. Quantitative results indicated that the experimental group achieved a significantly higher post-test mean score (M=8.25,SD=2.12) than the control group (M=5.80,SD=1.30) with t(98)=6.97,p<0.001, 95% CI [1.75,3.15], and a large effect size (d=1.39). Qualitative findings confirmed noticeable progress in students' ability to deconstruct scientific contradictions, construct evidence-based arguments, and make multi-dimensional decisions regarding real-world environmental issues.