And here I though that the bully / gang guy is actually decent with his suspicion to the summoner
then he decide to attack MC all of the sudden and not the old man that slash his forehead (what ! ?) and other student laugh at his skill (why ?)
Seems like another typical manga about OP Assassin guy who left the castle alone and become more OP ,getting harem and sh*t
The priestess has manipulation magic. She probably used it on him when she healed him in order to bring him over to her side, along with the rest of the people.
As for why they all hate the MC, they hated him even before the summoning, and his power is not only invisible to others, but also he's not telling them what it is so they assume it's weak shit and combine that with their existing judgement of him.
Yeah, it's cliche af, but it's also not the worst logic.