Urgh, this trope again.
Overused!!!
They didn't even bother to cross check his name or question why a commoner would have such expensive clothes. And don't tell me they marked his papers without his identification being found before that could be completed.
And what's with random people on the street all asking for his help? Not even in the middle ages would you ask random people on the street for help at every turn.
EDIT: Well nevermind, the mangaka chickened out in the next chapter. Instead of giving a proper lesson to Cain and the school staff, it's just waived with everyone laughing in good manner.