I posted this picture a while back for Les... I'm guessing you're Thomas will be pretty much the same as ours....
There are five screws in each window, three of them you get from inside the bus, they are in the upper glass slide channels. Lower the top glass, and look in the two side sliders, you'll see a screw on each side....then look at the top of the window opening, where the top edge of the window fits into the track, in the center there is a screw (you may have to pry back the seal material to access the screw.
The last two screws on the outside....as in the picture below....