A lightweight bus window construction has a window frame with a generally U-shaped channel around its lateral edges. A step is provided adjacent the opening of the U-shaped channel along the lower edge with a lip extending above the step along the edge. A thin sheet of plastic glazing material in the frame has its upper and side edges within the U-shaped channel along the upper and side edges, and the lower edge of the sheet rests on the step. The sheet is sufficiently thin that it will flex for fitting the upper and side edges into the channel without disassembling the frame. An elastomeric gasket fills the space between the sheet of glazing material and the face of the U-shaped channel on the opposite of the channel from the lip. The gasket includes a portion along the lower edge extending upwardly above the opening of the U-shaped channel and engaging the face of the sheet of glazing material opposite to the lip. The elastomeric gasket secures the glazing material within the frame.