Extending our findings on leaderboard-induced demotivation, this randomised controlled trial in 24 secondary classrooms compares cooperative team-based gamification with individual leaderboards. Gamification is widely adopted in secondary STEM education, yet competitive mechanics such as public leaderboards have been shown to reduce motivation among learners who prefer low-stimulation, reflective environments. This study tested whether a cooperative alternative, in which points are earned collectively by small teams and progress is displayed at the team rather than the individual level, produces better outcomes for introverted learners without disadvantaging their extraverted peers. Twenty-four Grade 9 and 10 science and mathematics classrooms in British Columbia (N = 612 students) were randomly assigned to a cooperative team-based gamification condition or an individual leaderboard condition for a ten-week instructional unit. Trait introversion was measured at baseline, and behavioural engagement, task persistence and unit achievement were measured at baseline and after the intervention. Multilevel models with students nested in classrooms showed a significant condition by introversion interaction. Introverted learners in the cooperative condition showed significantly higher engagement and persistence than introverted learners in the leaderboard condition, with effect sizes of 0.48 and 0.41 respectively, while extraverted learners performed comparably across conditions. Achievement gains were modest and favoured the cooperative condition overall. The findings suggest that cooperative gamification offers a design that is inclusive of temperament differences and that leaderboard mechanics should be treated as optional rather than default in STEM classrooms.