In this study, the authors aimed to develop lessons in which diversity and collaboration were ensured through children's cooperative involvement in a special class for students with intellectual disabilities. To bring out unique modes of expression for each student, the authors framed two practices and analyzed, as a lesson study, children's activities and the teacher's support in an unformulated communicative learning process. The authors found three key results. First, children's expressions deepen through a combination of both imitation and exploration, and collaboration emerged through this deepening process. Second, the pursuit of a teaching practice oriented toward diversity and collaboration contributes to the growth and development of both children and teachers, through discussing the learning environment, the teacher's actions, and the processes through which children explore their expressions. Third, children's motives for expression can be encouraged through intellectual curiosity and interest in musical instruments, the creation of an atmosphere where children can feel comfortable and the creation of positive relationships and a sense of partnership among the children and teacher. Their need for competence, then, can be stimulated by activities such as imitating each other's expression.