Objective This study is a prospective observational study on 75 late-adolescent survivors of a large passenger ship accident from immediately after the accident to one year later. Methods Assessments of student survivors were conducted on day 2 and at months 1, 6, and 12. The PTSD Checklist (PCL), Patient Health Questionnaire-9 (PHQ-9), State subscale of the State and Trait Anxiety Inventory (STAI-S), Athens Insomnia Scale (AIS), and Brief Resilience Scale (BRS) were administered. Results When the assessments for day 2 and month 12 were compared, all the scales, except the PCL-avoidance subscale, showed a significant improvement in symptoms among males. However, among females, all the scales, except the PCL-re-experience subscale and the STAI-S, failed to show a significant improvement. All the symptoms for both males and females showed a pattern that decreased to the lowest level at month 1 (camp-based controlled intervention period), then increased at months 6 and 12 (voluntary individual treatment after returning to school). Conclusion The rapid deterioration of psychological symptoms was found during the chronic phase, when students returned to their daily routines and received voluntary individual therapy. There is a need to screen high-risk adolescents and be more attentive to them during this period.