The one time I ever aggressively studied for all of my exams (in the bulk examination period at the end of a semester, rather than the tests or exams scattered through out a course) was some of the lowest grades I ever got. For me at least it seems intensive study leading up, or through, an exam period was a net negative - with the benefit of hindsight I assume due to similar causes to the last item in this list. Presumably each person has a limit beyond which spending more time studying results in declining performance in the actual examination, and part of earlier years exam periods is to find it.

Assuming you don’t have those asshole professors who brag about failing the bulk of the class - if the majority of your class fails, and does so regularly, you’re a failure as a professor. In a paid education environment that isn’t reasonable behavior.