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 that "jumped" ahead each minute, in a very noticeable 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.

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

Life does teach some lessons the hard way.

...from the Clean Jokes for a Dirty World website