At first glance, this story gives a false impression. It makes you think that the moral of the story is about being able to recognize the truly important things in your life, and knowing how to separate them from the less-important things. But that's all subterfuge.
See, after the pebbles, the students should've realized it was a trick question when he asked if the jar is full. They SAW that there was still space left for the smaller pebbles...so the next time he asked the same question, they should've seen it coming that something even smaller was going to fit in there. The REAL moral of this story is that this professor had a class full of morons. Fool me once, shame on you. Fool me twice, shame on me. Fool me over and over again with the same stupid trick, and I'm not qualified to be walking upright.