This paper explores the relation between formative assessment principles and their analogues in video games that game designers have been developing over the past 35 years. We identify important parallels between the two that should enable effective and efficient use of well-designed video games in the classroom as part of an overall learning experience organized and facilitated by teachers. We describe the parallels and then show how game-design elements are used in the service of formative assessment principles in Mars Generation One (MGO): Argubot Academy™, a video game developed by GlassLab that focuses on formative assessment of middle school students' argumentation skills. MGO was designed and developed together with a broader curricular unit on argumentation within which the game was situated. We discuss how design elements in the game satisfy each core formative assessment principle and how the game is connected to the instructional unit, demonstrating this marriage of game design and formative assessment. Prior work has reported on preliminary evidence on efficacy of the game when used as part of this middle school language arts unit.