In recent years, most studies on flipped classrooms or flipped learning have focused on learners' motivation, academic achievement and teachers' and students' perceptions of the flipped classroom. Little research has involved interactions in the flipped classroom. This study aims to show that in-class collaborative learning in the flipped classroom can not only be conceived as a collective activity consisting of discrete actions, but also as a systematic activity. From the activity-theoretical perspective, contradictions were found between components of an activity such as subject, tools, rules, community, division of labor, object and outcome. This study conducted a pre-experiment, selecting one lesson of the seventh-grade Information Technology Course as the learning content, conducted in a flipped classroom. Three triads with different compositions were specifically observed by recording their interactions. More second-level contradictions existed between subject and tools, and division of labor within the low-level triad than within the other two triads, and more first-level contradictions also occurred within the low-level triad than within the other two triads. Most second-level contradictions within the high-level triad were resolved by self-regulation of cognition. Instead, most second-level contradictions within the mixed-level triad were overcome by co-regulation or shared regulation of cognition. This study sheds light on how to effectively design and implement flipped learning, and provides a new approach for analyzing interactions in the classroom.