Various investigations have confirmed that pupils and teachers’ learning/teaching performance and health depend heavily on the quality and amount of daylight and indoor thermal conditions. The primary aim of using natural light in schools is to reduce energy consumption and costs, but it should also improve students’ performance. An appropriate configuration of windows improves visual and thermal comfort by reducing glare, distributing light and controlling solar energy gain. The study focuses on the impact of different transparency ratios (WWR) and window combinations in two critical orientations (west and east) on occupants’ comfort and the energy demands of a classroom. A building was selected for a case study investigation at the design stage. One east-facing and one west-facing classroom were studied. The two classrooms were simulated with the aid of the lighting and energy simulation programs DIALux Evo 6.0, DesignBuilder 5.5 and EnergyPlus 8.9. The results indicated that a glazing ratio of 50% would reduce the requirement for artificial lighting by at least 15% as well as providing more comfortable conditions.