This study examined the impact of process-based assessment on students’ self-directed learning abilities by applying it to performance practice classes with second-year students at G Arts High School over one semester. A mixed-methods design was employed, combining pre- and post-tests with in-depth interviews. The findings indicated that diverse assessment methods contributed to improvement in most sub-domains of self-directed learning abilities, while some domains showed no significant change, suggesting the need for a gradual, level-appropriate assessment design. Feedback also played a critical role in improving students' self-directed learning abilities, yet the study revealed the need for clearer feedback delivery and more refined assessment rubrics. While one-to-one private lessons effectively supported students’ musical growth, they were limited in fostering responsibility for learning, suggesting the need for a collaborative feedback structure between school teachers and private lesson instructors. This study is significant in that it provides empirical evidence for shifting the assessment paradigm in actual school settings through the implementation of process-based assessment. These results suggest directions for strengthening the connection between instruction and assessment in arts high school music education, and further provide implications for general secondary music education.