Why has social mobility declined?

Well, obviously it's the fault of teachers, according to evil Tory scumbags:

"Then earnest bureaucrats and ministers do worthy studies and conclude that people from less advantaged backgrounds have low aspirations for themselves and their children, and these low aspirations are a key (perhaps the key) factor holding them back. Well, go figure, guys and gals! You find a lot of vulnerable people and you preach to them that the world is unjust and they have no chance and then they end up with low aspirations. Whose fault do you suppose that might be??"

Preach? No, no, no, you vile complacent turd - I don't preach, I teach. I teach a subject to pupils who then take exams. Some do well and some not so well. Then they leave and take their chances in the real world where people's expectations are shaped by actual experience. "Whose fault do you suppose that might be?" This would be your fault - you and your friends, you nasty excuse for a human being.