[@KiritoAsuna] I'm not sure if I expressed the full prognosis here. What I'm really going for is an "bad guy vs. bad guy both using the innocents as weapons against the other" sort of deal. I figured the primary "character building" would be the manipulation of the students. With a good, detailed biography, static personality traits, and then some more transient, malleable traits on top of that, I felt that the students would be nuanced enough to essentially play themselves. 

Also, it seems that your concept of Demonic Possession is that the possessor has ultimate power over the possessed. This is not always the case. Many who claim to have been possessed also claim that their faith, as well as noticing the symptoms and signs of early possession, were enough to nip it in the bud. However, as kids, the students would have a decidedly feeble capability to resist, at least until they recognize it, the demon manifests, or it speaks to them. Oftentimes, in possession cases, the possessed undergoes drastic personality and behavioural changes without ever being aware that something is horribly wrong, until manifestation.

If I've misunderstood your point, I apologize. Some things just fly right over my head sometimes.