The present study assessed the effectiveness of the ECRIMO educational application designed to build first-grade level spelling skills. We tested whether using the app to teach spelling would be as effective as providing the same training using traditional paper exercises. The effect of integrating gamification into mobile learning apps, which has been little studied in the context of young children, is also investigated. A pretest/training/posttest design was implemented with 311 first-graders divided in four groups: no training, paper training, the ECRIMO app with gamification features, and the ECRIMO app without gamification. Spelling, reading and phonological awareness abilities was measured at both pretest and posttest. The training was conducted over a 7-week period (4.40 hours in total). The experimental design allowed us to answer three questions: (1) Is spelling training effective regardless of the medium used? (2) Is training through the app as efficient as paper-based training? (3) Does gamification impact students' learning performance? Mixed-model analyses revealed (1) a positive effect on the training outcome depended on the initial spelling ability of participants, (2) a comparable efficiency between autonomous training using the ECRIMO app on tablets and the same training provided by teachers using paper exercises and (3) a marginally positive effect of gamification that is greater for the weakest students. The present study proposes an original and pertinent experimental design to test the relevance of educational applications. The design features of learning apps can impact students' learning differently depending on their initial level. A critical step should be verifying that using online apps for training is at least as effective as the same training using paper exercises.