WMG / The Demon Headmaster

The Headmaster is a vampire.

Terrence Hardiman's prominent canines, the Headmaster's clear OCD and his hypnotic abilities all fit.

The members of Splat are part of the Headmaster's race
this is assuming that the Headmaster is an alien, it could also explain why they are immune to the hypnotism.

Rose was subjected to More Than Mind Control by the Headmaster.