An increasingly popular tool for improving engagement in collaborative learning is the addition of game elements such as leaderboards. However, the effectiveness of leaderboards promoting engagement is mixed. Users at or near the top of leaderboards tend to become more engaged while discouraging users at or near the bottom. Accordingly, this study uses grounded theory to discover design principles for leaderboards which lead to positive engagement in a video discussion. The results provide a theoretical model, Leaderboard System Engagement Model (LSEM), comprising four themes (clear goals, challenge/skill balance, timely feedback, and social influences) with multiple design principles for each theme. The model provides both designers and discussion leaders with valuable insights on improving collaborative learning via the addition of effective leaderboards.