The windows may have been “open”, but probably not wide open, and possibly variably open from one end of each group to the other. The airflow in the room couldn’t be so strong as to lift papers from the students’ desks. It would have to be kept to a feeble level.
So, based on the distribution of students that were affected by the virus, here’s my first guess:
My guess is that exterior air was generally moving into the room through the windows on the right, and out through the left. However, the exterior air that came into the room didn’t move straight across the room from one side to the other. Instead, it channeled towards the center of the room on the inlet side (right), and fanned out through the windows on the outlet side (left).
This weak airflow pattern would include two large oval-shaped vortices spanning the width of the room, one at the top and one at the bottom of the diagram, which I’ll call upper and lower. The upper vortex would be rotating clockwise and the lower one counter-clockwise.
There would be a stationary counter-clockwise vortex in the corner where the teacher’s desk was located, driven by the upper large vortex. If that’s where she was when reading to the class w/o a mask, the virus would have collected inside that corner vortex and from there mixed into the upper large vortex due to shear. That would lead to transport and recirculation of the virus from left to right along the front row of desks, past the seat in the center of the room, and from right to left in the central area, with some accumulation of virus in the remaining interior of the upper vortex.
In the middle of the room, the upper and lower large vortices would mix virus due to shear, transferring virus from the upper to the lower vortex. Some amount of virus, at a lower concentration, would then move from right to left in the center of the room, and circulate around the sides and bottom of the room along the perimeter of the lower vortex, moving left to right along the lower (back) wall. Also, some would accumulate, howbeit at a very low concentration, in the center of the lower vortex.
Some amount of virus would have been removed from the room by the global movement between the right and left window groups, but evidently not enough, especially if there were multiple instances of virus introduction by the index (teacher).
I have no idea how the door or other objects in the room affected the airflow. Also, the location of the air filter made it useless.
All of this of course, is a very crude assessment on my part, but something like this could explain the distribution of infections. A couple of floor-standing fans gently blowing air out of the windows on the left would have helped to disrupt this postulated flow pattern and keep the interior air cleaner.
I’ll add that an airflow pattern with natural global movement of exterior air from left to right through the windows is also possible, as are other patterns, but the same principles would apply.
I see no reason to doubt that the diagram, which shows all 25 of the desk spaces six feet apart, is incorrect.
