The rules at a particular university were such that if the professor
were not present in the classroom by 15 minutes past the hour, the
class was considered a "walk" and the students were free to leave --
with no penalties for missing a class.

The rooms were equipped with
the type wall clocks which "jumped" ahead each minute, in a very
noticable fashion.

As it were, these clocks were also not of the most
sophisticated construction.

Some enterprising student discovered
that if one were to hit the clock with chalkboard erasers, it would
cause the clock to "jump" ahead 1 minute.

So, it became almost daily practice for these students to take
target practice at the clock ( as it would have it, this particular
professor was not the most punctual, and the students considered
him severely "absent-minded").

A few well aimed erasers, and lo, 15
minutes were passed, and class dismissed itself.

Well, when the day for the next exam rolled around, the professor
strolled into the room, passed out the exams, and told them "You
have 1 hour to complete".

The professor then proceeded to collect the erasers from around
the room, gleefully took aim at the clock.

When he had successfully"jumped" the clock forward 1 hour, he closed the class and collected
the exam papers.