I know a lot of people are asking why get revenge on them using such a elaborate scheme. Can't Eunhyung just beat up the guy?
Here's what I think. Eunhyung and the rest are just 1st year and are ordinary students (i. e. No rank). If Eunhyung beats up that guy then he takes his rank. This will provide an opportunity for the rest of his gang to keep bothering him and his friends. But since they got the higher ranks to take care of it, the rest will have no choice but to shut up and walk away. The higher ranks are doing it as punishment for breaking the rules. Hence no one will dare to pull a stunt like this again and by doing this they have established that they have the higher ranker's support.