Speaking is one of the essential competencies in English language learning; however, developing this competency in the classroom requires strategies that can encourage active student participation. Small Group Discussion (SGD) is considered a strategy with the potential to meet this need, but its effectiveness depends heavily on how the teacher implements it. This study aims to (1) examine teacher’s implementation of SGD in teaching English speaking skills, covering the planning, implementation, and evaluation stages based on the five principles of Cooperative Learning by Johnson & Johnson; and (2) identify the challenges the teacher faces during the process. This study employs a qualitative approach using a single-case study design conducted at a public high school in Central Java. Data were collected through classroom observation, semi-structured interview, and analysis of the teaching module document, then analyzed using the interactive model by Miles, Huberman, and Saldaña (2014). The findings indicate that the teacher implemented SGD systematically, with the five principles of Cooperative Learning reflected in classroom practices, although group processing was not yet carried out optimally. Key challenges included uneven student grouping, time constraints, and difficulties in assessing individual contributions. The limitation of this study lies in its single-case study design, which involved one teacher at one school. Therefore, the findings cannot be generalized.