Most users of educational technology disengage within weeks of joining, and whether temporary incentives can durably change engagement remains unclear. We study contest-based gamification in a randomized controlled trial with 10,000 students on an English-learning app in India. Treated students entered a reading contest with leaderboard rankings and prizes for top performers. During the contest, treated students completed 45% more stories than control (0.96, SE 0.32). Twelve weeks after all incentives were removed, treated students completed 76% more stories (0.30, SE 0.14), relative to a control mean of 0.40, and the share of active users rose from 6.5% to 9.6%. Effects are concentrated in the upper tail of the engagement distribution, where nearly all platform activity is generated. Each story completed during the contest generates more lasting engagement for treated than for control students, and persistence extends beyond students who received prizes. Temporary contest-based incentives can durably expand the small segment of engaged users on which educational technology platforms depend.