The revised course of study guidelines for senior high schools in Japan now features a new subject, “Logic and Expression”, designed to enhance communication skills through the formulation and elaboration of logical structures. This addition underscores a rising societal emphasis, both locally and globally, on fostering the ability to articulate thoughts logically and convincingly, anchored in critical thinking (CT). Existing research reveals a positive correlation between CT and speaking skills, crediting English debate, pair practice, and peer assessment as effective strategies. This study seeks to examine the impact of utilizing the “format” in debate instruction and implementing pair practice to augment high school students’ speaking skills and CT. Interviews were analyzed through the lens of the Modified Grounded Theory Approach to derive our core hypothesis: the instruction of debate “format” exhibits a synergistic relationship with pair practice. This research endeavors to illuminate the mechanisms through which debate “format” instruction and pair practice influence the growth of CT and speaking skills from the learners’ perspective. It underscores the value of collaborative learning in a secondary educational environment, projecting potential applications across diverse speaking instruction practices.