Artificial intelligence (AI) literacy has emerged to equip students with digital skills for effective evaluation, communication, collaboration, and ethical use of AI in online, home, and workplace settings. Countries are increasingly developing AI curricula to support students' technological skills for future studies and careers. However, there is a lack of reviews summarizing AI education in secondary schools. This study aims to provide an up-to-date summary of the literature, informing researchers, policymakers, and educators on fostering students' AI literacy. A thematic analysis of 50 AI education studies from 2016 to 2022 was conducted, evaluating pedagogies, teaching tools, learning contents, and assessment methods among secondary students. Results were identified: (1) Collaborative project-based learning, involving interdisciplinary problem-solving through artifact creation, emerged as the most common pedagogical approach; (2) Teaching tools were categorized into hardware, software, intelligent agents, and unplugged tools; (3) Junior students focused on experiencing AI and basic concepts, while senior students explored advanced and technical components; (4) Assessments included knowledge tests, questionnaires, and qualitative analysis (e.g., videos, documents, presentations); (5) Students' learning effects were measured across affective, behavioral, cognitive, and ethical dimensions. The study identifies suggestions and challenges for implementing AI education in secondary schools, offering valuable insights and recommendations for educators and decision-makers.