As high school students approach graduation, they face important decisions about their future, yet many lack formal training in decision-making. To address this gap, the "KLUGentscheiden!" (KLUG) intervention was developed to teach students how to make complex decisions. KLUG focuses on three core components of effective decision-making: defining personal objectives, identifying relevant alternatives, and comparing alternatives through weighted evaluation. In this study, 157 students participated in a pseudo-randomized controlled trial, with the intervention group receiving two days of KLUG training, supported by the KLUGnavi.app for career planning. Training success was measured via career choice motivation (CCM) and career choice self-efficacy (CCSE) before, immediately after, and 8-11 weeks post-intervention. Results showed significant improvements in both CCM and CCSE for the intervention group, with sustained effects at follow-up. These findings highlight KLUG's potential to enhance decision-making skills in high school students, improving their readiness for post-graduation choices.