This study investigates the interaction between thermal mass and natural ventilation in free-running primary school classrooms in a hot-humid tropical climate. Field measurements from six paired Interlocking Compressed Earth Block (ICEB) and Sandcrete Block (SCB) classroom sites in Anambra State, Nigeria (July 2024; n = 257 pupils) confirmed a statistically significant mean indoor temperature advantage of 2.03°C (t = 6.83, p < 0.01, Cohen's d = 2.79) in favour of ICEB construction. A calibrated dynamic thermal simulation (DesignBuilder v7.3/EnergyPlus; ASHRAE Guideline 14: SCB CVRMSE 13.5%, NMBE −12.6%; ICEB CVRMSE 6.3%, NMBE −6.3%) extended the evidence to dry-season conditions. Results demonstrate that high thermal mass moderates indoor temperature fluctuations during the wet season but that its effectiveness under dry-season harmattan conditions is diminished by continuously open screen-wall ventilation, which introduces sustained heat ingress and limits nocturnal thermal purging. The paper proposes a conceptual framework for Adaptive Thermal Mass (ATM) infrastructure, in which controllable natural ventilation is treated as an operational component of passive thermal regulation rather than an independent design strategy. Although grounded in a tropical case study, the ATM framework has broader relevance for regenerative urban infrastructure facing increasingly variable climatic conditions, including the overheating challenge in UK educational buildings.