It really depends on what kind of class it is, but at most schools:

* If 1/3 of calculus physics for engineers fail, they take it next semester.

* If 1/3 of gen-ed physics for poets fail, the professor better have a good explanation for the provost.

* If 1/3 of physics for pre-meds fail, the professor better have a pretty good home security system and might want to think about having the family stay in a hotel for a few weeks.