Indoor environmental quality (IEQ) of school buildings significantly affects the well-being, health, and academic performance of pupils. This study evaluates various aspects of IEQ in high school through objective measurements and subjective assessments by pupils and teachers. Real-time monitoring was conducted to measure thermal comfort parameters, air quality, and pollutants such as CO₂ and volatile organic compounds (TVOCs). In addition, a questionnaire survey was used to determine perceptions of IEQ-related issues including noise, temperature fluctuations, and air quality problems. The results showed that although thermal comfort parameters generally met legislative requirements, elevated concentrations of CO₂ and TVOCs were found in all classrooms, indicating inadequate ventilation. Survey responses indicated that pupils often experienced fatigue and difficulty concentrating, which could be related to poor air quality. These findings highlight the need for improved ventilation strategies and better management of indoor environmental conditions to create a healthier and more conducive learning environment.