Teachers have used games as a support tool to engage students in learning tasks. As they often record student's performance as learning progresses, it is interesting and useful to discuss how that information can be used to assess learning and to improve the learning experience. For instance, teachers can use that information to give personalized attention in classes and the game can use it to provide challenges of the "right" difficulty. In computer programming learning, games can provide an alternative way to introduce concepts and, mainly, to practice them. This paper proposes a model to identify the students' progress considering their performance in programming tasks. The model is demonstrated by an implementation in a casual computer programming serious game. We illustrate how this game could use this model to personalize its challenges.