Background: Online learning has become a popular form of education. Prior to the COVID-19 pandemic, online learning was mainly associated with higher education, with an incremental growth at the K-12 level. The pandemic changed this situation rapidly. Online instruction has been increasingly integrated into secondary schools and has significantly impacted students' learning experiences and outcomes, particularly for high school students experiencing the critical transition to higher education. Therefore, there is an urgent need to develop an in-depth understanding of their perspectives through a synthesis study to provide insights into critical factors influencing their online learning. Objectives: Following the PRISMA guidelines, this article systematically reviewed 46 empirical studies that examined high school students' perspectives through their experiences of taking at least one online course in synchronous and/or asynchronous learning environments by leveraging diverse technologies. Methods: We extracted and summarised results from studies and applied descriptive statistics analysis and thematic analysis to identify and synthesise the major patterns/themes of findings. Results and Conclusions: Students perceived the main benefits of learning online, ranging from their increased learning autonomy and communicative flexibility to their skill development across subject content areas. They identified several challenges in taking online courses, including staying motivated, lacking instructional support and social engagement, and time management issues. These findings suggested that educators and stakeholders develop evidence-based and 'smart' designs of online learning environments and course structures, given careful consideration of the needs and characteristics of intended student groups and content to be delivered.