This study demonstrates that heat disproportionately impairs human capital accumulation among low-performing students compared with their high-performing peers, using nationwide examination data from 22 million students in Japan. Given the strong correlation between academic performance and socioeconomic background, this suggests that heat exposure exacerbates pre-existing socioeconomic disparities among children. However, access to air conditioning in schools significantly mitigates these adverse effects across all achievement levels, with particularly pronounced benefits for lower-performing students. These findings suggest that public investment in school infrastructure can help reduce the unevenly distributed damage caused by heat to student learning, thereby promoting both efficiency and equity.