Ari is Jeb's biological son, and although Jeb does work at the school they turned Ari into an Eraser, and he pretty much is the leader of them. Since Jeb left with the flock when Ari was three the school had extra time to experiment on him, which turned him into the Eraser that he became. It seems there is a bit of jealousy that Jeb paid more attention to Max and the flock and Jeb just left Ari there without his Dad to watch over him.