Doesn't help that he's got the situation all wrong. Its a single iteration of a game that has a simple payoff matrix if anybody cares to notice it.

There is no true 'commons' here, its just a situation where the payoffs are skewed to promote 'defection.' Nobody loses anything that they already had in this case, since the 'commons' goes away after the decision in either case. There's nothing to 'preserve' for the future.

Its still completely possible to coordinate around this and hell, if you like you can ensure that 9% of the students (those who demonstrate the need, I guess) can get 6 points while the rest get two, if they manage to get everybody (or enough of everybody) on board.

A true tragedy of the commons would something like "there are 10 extra credit points up for grabs every day, any person can claim as many as they like every day, and the only catch is that if they are all claimed in any one day, then no more are available ever. If there are any remaining at the end of a day, then 1 point will be added, then the total will be doubled, to a maximum of 20 available points for the next day."

Then they'd have to coordinate around the depletion of a scarce resource that could be maintained and 'cultivated' if they wanted to maximize 'societal' gains. They could cultivate a maximum of 11 points a day, but if any one person 'overuses' the points then they are reduced or even depleted entirely.

IF YOU DON'T ITERATE THE DILEMMA, ITS NOT A TRUE TRAGEDY OF THE COMMONS in any real sense. In his exercise, the points are either gotten or they are not. Its not a situation where some DO gain a serious benefit and the rest do not. Which, obviously, is unfair in a classroom setting.

Just a suggestion, as that sort of arrangement properly demonstrates the point he claims to be making.