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Good daylighting design in buildings not only provides a comfortable, luminous environment, but it also helps save energy and create a comfortable and healthy environment for the occupants of the building. The importance of receiving daylight in educational buildings is due to the time synchronization of operating hours of the school, and daylight plenitude in the sky. Daylight improves environmental quality and saves energy by minimizing artificial lighting requirements and, thus, reducing the cooling load. Furthermore, window design is a primary sustainable approach to achieving environmental goals and contributes to comfort and satisfaction. The present study analyzes the variations of annual thermal energy demand and useful daylight illuminance to determine the optimal Window-to-Wall Ratio (WWR) of a simulated primary school classroom in Tehran in two directions of the south and the north. Twelve different window sizes in two types of horizontally expanded windows (the height of the window remains constant, the width is varied) and vertically expanded windows (the width of the window remains constant, the height is varied) were compared from the two perspectives of Useful Daylight Illuminance (UDI) and thermal and electrical energy using the Honeybee plugin of Grasshopper. The results of this research demonstrate that vertically expanded windows are more energy-efficient in classrooms while they give almost the same UDI as horizontally expanded windows in both south and north facades. Overall, WWR in the range of 25% to 35% in the south facade displayed the best performance in daylight and thermal loads. [ABSTRACT FROM AUTHOR]