Fake Heir? I Rule the Elites!

At an academy for the children of the rich there is a rule about how fish is eaten at table. The people who belong eat the meat from the neck, and eating from the back marks you as poor. One student takes a big mouthful from the back of the fish and is written off as a poor boy with no business being there. The sons of the four biggest families at the school make him their plaything. He goes along with it for a while. Then he turns up among them as the heir of a billionaire family and starts on the school's rules, breaking them one at a time in front of the people who set them. The ones with the most power at the school are humiliated in turn, and the heirs who mocked him start competing to get close to him.