Fanon: Many people have interpreted the eponymous character as an alien, in no small part due to his hyponotic abilities and the fact that his eyes are green.
Moral Event Horizon: we all know that Rose, being the de-facto leader of the Prefects is strict and all, but she crosses it when she punishes Lloyd, Harvey and Dinah by making them shovel snow (without their coats on), and to top it all off she and the prefects push the trio in the snow not caring about Harvey's asthma.
Paranoia Fuel A good example for a 100+ page kids book actually, is the Headmaster's power. Unless you're one of the very select few who can resist him, you end up doing everything he says even if you don't want to and you won't know why. All you would know was that you saw a innocuous, kinda funny looking fellow who you may remember falling asleep near. We see it first hand from Dinah's perspective in the books.
What's worse is that he's able to use technology to the same end, meaning you don't have to see him.
Well-Intentioned Extremist: The Headmaster's schemes are focused upon creating and maintaining order and efficiency...at the expense of democracy and freedom.
What an Idiot: In Takes Over it turns out that the Headmaster, being a clone of the original, has lost many of his memories, including what his name is, which prevents him fully taking control of the Hyper-Brain computer because it wants to know his name before giving him full access. Instead of just making up a new name for himself he comes up with a convoluted scheme to lure the SPLAT members to the laboratory so that they can tell him what his name is. Naturally, this leads directly to his defeat at the hands of the group. It's a little less blatant in the TV series, where he knows that Dinah isn't likely to directly give him the information that he wants, and so his intention is to have the Hyper-Brain absorb her mind and then obtain the information that way.