I'm not OP, but I think his question is more, "Why does school set people up to be disappointed by the jobs available to them?" Not, "Why does school teach things that aren't necessary on a job site?"
The ambiguous intention behind the question has spawned two separate discussions in this thread, one of which isn't that interesting because I think we all agree (that there's more to life and education than employment).
If I'm right and his stance is the first, I'm sympathetic to it. The system, while training people for jobs better than what will actually exist for them, isolates people from its most broken aspects (workplaces), promising betterment and skill advancement, during the age of peak revolutionary energy. By the time they figure out what employment really is--for most people, an exploitative and unforgivable waste of their time--they are already in their late 20s, and any revolutionary movement has been deprived of their most energetic years.