National standards show poorer children do worse. Why?

Why are children from poorer backgrounds less likely to perform well in reading, writing and maths?

Data released at the weekend shows that pupils who attend low-decile schools will more likely perform below national standards than those at higher-decile schools.

Principals Federation president Paul Drummond told Marcus Lush that the data doesn't offer anything that wasn't known four years ago and falls short of finding a solution for the disparity.

Are national standards helpful?