As said before, Alex was the rising star of the human side, with potential similar to Vanessa's—something people discovered quite late to do anything, just like they did with Vanessa.
Now, with Alex in the academy, they couldn't kill him. Not because it was forbidden to kill anyone in the academy city—it happens a lot; people just don't know about it—but because Alex was an invited student.
Students with invitations are those who come under the protection of the mysterious academy's principal. A being with unknown but unfathomable power.
So, unless Alex was out of the academy city or he had taken part in a life-and-death match, there was no way to kill him in the whole of Originveil City.