It will never happen that way. If everyone in a class fails the first exam, most likely the instructor is going to back off on the second. If the whole class gets As one semester, the next semester the course is going to be more difficult. The extent to which such adjustments are made will vary greatly, but they will always be made. Of course if an exam has been developed through testing or over the years, such adjustments may not be necessary for a long time or may be done during the exam development phase.
Also, if grades are clustered bimodally or anti-normally, then the difficulty range on questions probably needs to be adjusted. Of course, if it is a class of fifteen students or something, then skill distribution of the students may be far from normal, but for a thousand students or so you should expect something resembling a normal distribution.