The current study aimed to create and validate a digital literacy assessment tool's quality for students in Grades 10-12 within the Thai educational context, and 2) to develop T-score norms derived from the results of the digital literacy assessment tool for students in Grades 10-12 in this context. The study followed a research and development (R&D) approach, including content validation, pilot testing, and confirmatory factor analysis (CFA) for construct validation. The participants consisted of 1,590 Grade 10-12 students from schools under the Phetchabun Secondary Educational Service Area Office, Thailand. Content validity was assessed using the Index of Congruence (IOC), and construct validity was verified using confirmatory factor analysis (CFA). Item difficulty, discrimination indices, and reliability (KR-20) were also analyzed. The results showed that the assessment tool demonstrated strong content validity (IOC = 0.60-1.00), acceptable difficulty levels (0.20-0.80), and discrimination indices (0.22-0.74). CFA confirmed the six-component model with excellent fit indices. The tool's overall reliability was 0.94, with component reliability ranging from 0.70 to 0.83. T-score norms were developed to interpret student performance. This study provides a systematically validated digital literacy assessment tool tailored to the Thai educational context, supporting effective measurement and development of students' digital competencies.