A prefabricated wall panel has a top frame structure defining an upward-opening channel extending lengthwise along the upper edge of the panel. An elongate header member is normally disposed within this channel. The header member is vertically slidably supported on supports fixed to the top frame structure so that the header member, in a storage position, is retained in the space. When the panel has been swung upwardly so that the upper edge is aligned below a preinstalled ceiling track, then the header member is manually displaced upwardly until the header telescopes over the ceiling track. A fixed connection is then formed between the header and the ceiling track to provide lateral stability at the upper edge of the wall panel.