One LLM Does Not Simulate All Students: Ability-Aware Student Simulation via Cognitive Diagnosis Guided LLM Assignment

Large Language Models (LLMs) have become integral to personalized education systems, particularly in the realm of student behavior simulation. By predicting fine-grained learning behaviors, these simulations enable intelligent systems to provide tailored instructional support. However, most existing methods rely on a single high-capacity LLM to represent an entire population of diverse learners. In this work, we demonstrate that this "onesize-fits-all" approach induces a systematic ability-dependent bias, where high-capacity models tend to overestimate low-ability students while lower-capacity models underestimate high-ability ones. To mitigate this distortion, we propose an ability-aware student simulation framework that dynamically matches students with appropriate LLM backbones through cognitive alignment. We leverage Neural Cognitive Diagnosis (NeuralCD) to extract multidimensional cognitive profiles for both human students and LLM agents within a shared skill space, subsequently pairing each student with the most cognitively representative model. Extensive experiments demonstrate that our approach substantially reduces simulation bias and consistently outperforms single-model baselines across the entire proficiency spectrum. Our findings suggest that faithful behavior simulation necessitates the alignment of model capacity with student ability, establishing cognitive diagnosis as a principled mechanism for model assignment in educational AI.