I remember watching this from a very young age, and being freaked out by it. Its darker then the usual kids program, with clever scripting and threatening themes. The villian himself is played to full icy, formal british evilness....while the three main kids performed their roles with great gusto. Its typical, old fashoined fun....the kids against the grown ups, while its obvious who we're rooting for. It also rebels against the idea of a uniformed society, where formalness rules supreme. It heralds an important message and is much more gritty then the sugar coated fluffball kid's TV we see today.
The one thing I remember watching when I was younger. The opening music is creep, and the Headmaster is terrifying!
I recently purchased the copied DVDs off ebay and watched it again. And it's still pretty good. The effects are a bit dodgey, and the children have a weird habit of saying brilliantly written dialogue, before grabbing some food and proclaiming that they are "famished". I don't remember anybody talking like that in the 90s!