I sort of see why high school teachers do this. Often, as adults in the work force, we are put into work situations where someone isn't pulling their own weight. It's part of life and life isn't always fair. It's how you learn to deal with these situations.
Let your son know how proud you are and ask him what he's learned from this experience. If he's felt he has not learned anything, perhaps you can guide him towards the real "lesson".
You can't make other people be responsible, but you can take resonsiblity for your own actions
Sounds like you got a good kid there!