Metacognition in L2 listening has garnered attention, but there is limited research on concrete methods to practice it in high school EFL contexts. This study aims to investigate the effectiveness of self-annotation interventions in a high school EFL listening classroom. Through the analysis of pre- and post-listening tests, students' annotations over 10 sessions, and a post-questionnaire, this study reveals that self-annotation interventions improve students' comprehension, confidence, and motivation. Both higher- and lower-level students demonstrated improved L2 listening test scores compared to the control group, with the former generating more analytical self-annotations and achieving more statistically significant progress. The thematic analysis of students' self-annotations identified recurring patterns that can inform effective listening instruction, while also enabling students to recognize and correct their mistakes, as well as receive cognitive and affective support. This study emphasizes the importance and feasibility of incorporating self-annotation in high school EFL listening classrooms, making students' reflection processes observable and actionable, and providing recommendations for effective metacognitive interventions.