What brings the scholastic ability gap?

The Ministry on education made public that there is serious ability gap between schoolchild of high salary family and of low salary family.This gap may bring about the gap in the future.Why has this happened? I think this is because children run short of motivation to study. Why do we study? Teachers should have answer to children question"Why do we study?" I should answer,"We study hard to be happy and to make other people to be happy."