This study investigates the degradation of indoor environmental performance in a colonial-era school building under current tropical climate conditions and its implications for occupant comfort and cognitive function. A field study involving 30 students over 18 days was conducted, integrating environmental measurements, subjective responses, and cognitive tests (attention and working memory). Results indicate that indoor air temperature, relative humidity, and illuminance exceeded recommended standards, resulting in elevated PMV (≈1.5) and PPD (>50%), reflecting thermally uncomfortable conditions. Statistical analysis revealed significant negative relationships between thermal stress indicators (PMV and HSSI) and cognitive performance (r up to -0.94), demonstrating that increased thermal load reduces attention and working memory accuracy. In contrast, lighting showed no significant effect, likely due to uniformly low illuminance levels. These findings suggest that passive design strategies in colonial buildings are no longer sufficient under current climatic conditions and may compromise learning performance. The study highlights the need for adaptive design interventions to maintain indoor environmental quality in tropical educational buildings.