Installing permanent screening in the little vent windows inside the full sized window is fairly easy but to put a full sized screen on the entire exit window gets harder because you have to remove the inside screen in order to unlatch and open the window (assuming your windows are like mine).
The easiest way I can think of is to remove the inner clamp ring (make sure the window doesn't fall out) then use silicone sealer around the window opening to glue screening to the bus body then remount the clamp ring. Problem with that is you can't unlatch the window to open it with the screen in the way. A solution to that might be to make some access flaps out of screen and velcro or magnets so you can open the screen near the latches and get your hands through to open/close the window.
You could also sew loop velcro around the outside of the screen then use hook velcro strip to mount it onto the window frame. That would make a screen that's easy to mount, open and roll up and store. It's got to be something easy to do or you just won't open the bloody window.
Magnets would be a natural solution if only the window frames weren't made of aluminum.
There are lots of people here that are good a thinking outside the box. Someone will come up with an elegant solution.