Your windows are aluminum, no? It would be a tough order to weld them to the steel ribs.
A number of my windows leaked (and are still leaking) in this manner. The source (in my case) is the seam between the bottom part of the window's outer frame and the two side parts. These pieces are only pressed together, so if this gap opens up even a tiny bit, it will allow in substantial water (when I had my windows out and put a hose on this part, water sprayed through into what would be the inside of my bus when mounted). The fix is to seal the outside of the window sill, in the hard-to-access corners that the moveable pane slides down into. I used my pinky to spread the seam sealer here, but it has not adhered very well and I need to redo my sills with a small craft brush.