In this exploratory study, we designed and validated game-based performance tasks to assess the development of representational flexibility in autistic adolescents through virtual reality (VR)-based training. Representational flexibility, a critical cognitive ability, involves attention switching, generating representations, and recognizing patterns--skills essential for adaptive problem-solving and success in STEM education. Despite the increasing use of VR in educational interventions, limited research has explored dynamic, real-time assessment methods for representational flexibility, particularly for autistic learners. To address this gap, we conducted 71 VR-based training sessions with seven autistic adolescents, implementing a variety of game-based tasks targeting specific subsets of representational flexibility. Using both correlation and regression analyses, we evaluated the predictive validity of these tasks. Findings indicated significant relationships between task performance and representational flexibility subsets, with certain tasks demonstrating strong potential as proxies for dynamic cognitive assessments. The study highlights the viability of game-based performance tasks as an innovative approach to tracking cognitive development in immersive learning environments. Broadly, these findings contribute to advancing inclusive VR-based educational practices and assessment methodologies tailored to the unique needs of neurodiverse learners.