One example of a one liner graduation joke is "If you think your teacher is tough, wait till you get a boss." This joke implies that bosses are harder to work for than teachers are.
Another example of a graduation joke is "Commencement speeches were invented largely in the belief that outgoing college students should never be released into the world until they have been properly sedated." This is actually a quote about graduations from Garry Trudeau. This joke suggests that most graduation speeches are boring and that the people that invented them purposefully to put the graduates to sleep instead of encouraging them.