Not an urban myth - that little joke was pulled more than once in US grammar schools in the 70/80's
Personally, I don't think there is a lesson to be learned here. It amounts to nothing more than a lame practical joke.
The one we had was a set of increasingly complex instructions, and the last instruction saying to just sit there quietly. So you would be working hard at solving the problems, and various people would just be sitting there with a smile on there face. You knew these people, since they were your classmates, so they weren't savants, so how did they get through these problems so quickly. Eventually each one of you realise, and look for the trick, getting to the last question. There is always someone who just keeps going.
2) Even if it did teach you to follow instructions, is that a good lesson? I think I've only gotten to where I am in life by explicitly NOT following instructions.
#3 Just happened to be the toughest material of the class, where a mastery of both Haitian witchcraft and the Coriolis effect were needed.