Some researchers indicate that the transition to high school deflects adolescent developmental trajectories. Others assert that it provides a new possibility for the promotion of adolescents' socioemotional well-being. One critical view missing in such claims is that individual variabilities interact with environmental influences. We employed the framework of Differential Susceptibility Theory, which postulates that individual susceptibilities moderate external influences "for better and for worse." To clarify the mechanism of adolescents' differential adjustments, this article investigates the role of sensory-processing sensitivity using the Japanese version of Highly Sensitive Child Scale for Adolescence (J-HSCS) and tests whether the diathesis-stress model or the differential susceptibility model best describes students' socioemotional adjustment across their high school transition. The current article used the two-wave data collected from Japanese adolescents aged from 14 to 15 years (n = 412, 50% girls). In Study 1, we investigated the replicability of psychometric properties of J-HSCS. The results supported previous findings, indicating its validity for the bifactor model. In Study 2, we utilized confirmatory competitive model testing, which maximizes statistical power by parameterizing the crossover point to allow a direct comparison of alternative models. The results indicate that neither the diathesis-stress nor the differential susceptibility models fitted the data. Rather, a strong vantage sensitivity model was revealed, suggesting that highly susceptible adolescents disproportionately benefitted from a positive school transition over their counterparts. This finding signified the role of adolescents' sensitivity to environmental influences and the importance of considering its moderation under Person × Environment interactions.