A window module for a school bus includes a frame, a fixed window panel affixed to a portion of the frame, a perimeter panel secured to another portion of the frame and defining an opening at the other portion of the frame, and a movable window panel. The movable window panel is slidably positioned at the frame such that opposite perimeter edges of the movable window panel are slidably engaged with opposite channel members of the frame. The movable window panel is movable along the channel members and interiorly of the fixed window panel when opened and interiorly of and generally at the perimeter panel when at least partially closed. The exterior surfaces of the fixed window panel and of the perimeter panel form a generally flush surface at an exterior of the school bus when the window assembly is installed at the school bus.