It made no sense for me for the teacher guy or the Oni organization to be so selective of their peers. You are in an existential war against another organization, so you need the numbers. Also, the protagonist just discovered he's an Oni and lost a father figure in front of his eyes, so how the hell do you expect him to be able to control his power from the get-go? I would've understood that if it were a test to see if the protagonist would join some elite unit, and there they only accept geniuses.