Unfortunate duo

Kio and Shiro are twin boys living in a world where classification was most important, it didn't matter how rich you were, how poor you were, how messed up your situation was but everyone must get classified at one point. There are five categories: dominant being the highest then sub, pet,slave and littles being the lowest category due to them being the most helpless out of the 5.
The twins have already been brought up and raised in a harsh environment and were taught from young that anything lower than a dom category from either of the two deserves a beating of a lifetime before getting disowned they have been taught that 'weakness is never an option'

So what will the boys do when they get the dreaded letter on their classification day at school?