A visit to the Schoolbook Depository shows that the schoolbooks are not stacked one at a time but stacked neatly in a great many boxes, and they are still in place; no tripping over them is likely.
Also, Oswald was working on that floor, or pretending to work there, which normalized his presence at the infamous window for as long as he desired.
Some of the boxes were stacked in front of the open window in such a way to firmly support the muzzle of the rifle, greatly facilitating marksmanship.
The car had completed the short entry curve to the parkway and was proceeding fairly slowly in a short straightaway with its rear toward the window, meaning the left-to-right aspect ratio for the shot was minimal, only the changing range elevation from shot to shot was an issue, and a minimal issue at that.
The actual range was remarkably short, leaving a person standing at that window today with the clear feeling that the shot could have been performed with accuracy using a decent slingshot or a bow and arrow. A crossbow would have been a sure thing. A reputable rifle would have made the shot rather hard to miss. The target was that close.
Working the bolt action requires time, but there was plenty of time to work the action three times before a car completed that short straightaway and began turning right to follow the roadway, making the shot much more difficult. And only a single shot on target would have been sufficient.
In short, the shots were not only possible, they were noticeably easy. The average person could have been trained to perform them with less than an hour of training and practice. Remember, the rifle was fully supported by boxes, and the boxes are still in place.
Visit the building, and it all becomes readily apparent.