If we step back from the camera position of the last shot, we have an opportunity for a panorama of the old building's south elevation.
The structure between the old building and the chimney is the old gymnasium. With the advent of the new building and the associated gymnasium, floors were installed within the old gym. The lunch room occupied the new lower floor and classrooms were built in the new space upstairs. If you recall, to enter Mercer's room and the library, one had to walk down a ramp because the elevation of the new floor in the top of the gym did not agree with the elevation of the third floor of the old building.
Mercer's room has now become part of the library. Something has not changed, the floors still bounce as you walk around!
