Group improvisation has been described as a learning context in which participants negotiate musical meaning through interaction with others. This study examines how encounters with heterogeneous co-performers, that is, musicians who differ in background, genre, and expressive approach, affect novice students with limited improvisation experience. Such encounters are expected to enhance musical expression and encourage openness to diverse approaches, with potential educational implications. The participants were 22 undergraduate students enrolled in a liberal arts course, Children and Music, at Yokohama National University in Japan. Two class sessions focusing on group improvisation were conducted: the first class session only involved students, and the second class session incorporated two professional musicians as heterogeneous co-performers. Students completed post-session questionnaires and participated in semi-structured interviews that focused on their musical expressions, engagement, and experiences of interacting with the heterogeneous co-performers. The data were examined in relation to the students’ musical backgrounds and attitudes toward improvisation. The findings showed varied reactions to the presence of heterogeneous co-performers. The study paid particular attention to two students (Student A and Student B), whose interview responses contrasted clearly and offered detailed reflections on their experiences. Student A, who had extensive experience in classical piano, described the sessions as an opportunity to explore new expressive techniques. In contrast, Student B, who preferred structured and harmonious music, experienced anxiety and discomfort, perceiving the heterogeneous co-performers’ approaches as disruptive. The two cases suggest that students’ prior musical experiences and personal tendencies influence how they interpret and respond to heterogeneous musical expressions. These findings suggest that introducing diverse performers can challenge students’ existing frameworks of improvisation, while also requiring careful pedagogical support. Educators need to create learning environments that provide both emotional security and openness to diverse forms of expression. Such environments may facilitate encounters with heterogeneous co-performers, broaden students’ perspectives on music, and support inclusive approaches to music education.