You might think that a class-based capitalist society is the best option available, but that's a far cry from claiming that it is just and non-oppressive. The simple fact is that the statistically most likely way to be a rich person is to be born into a rich family. No one is saying that it's the only way, but if you want to lay odds its the way to bet. That's just? It plays havoc with the idea of a meritocracy.
The lesson that I understood that the kids were learning is that what you happen to have started out with doesn't give you inalienable rights. In other words, be sensitive to the fact that you got where you are _at least partially_ by luck. The rest is just figuring out what it means to be sensitive in this way.
There is another important lesson that I think the teachers missed (about how to handle the disenfranchisement that the Lego-less kids felt), but that's another story.