Like many impressionable adolescents within late-night or Saturday-afternoon viewing range of WOR (Channel 9) or WPIX (Channel 11) in the mid to late seventies, I developed an early affection for this, the perfect adolescent horror clunker. Yes, the papercutter. Yes, the acid vat. Yes, the cleats. And hell yes... Mister Mumps. Whatever Pat Cardi's shortcomings as an actor, he made a convincing tortured high-school Jeckyll & Hyde. I never found it particularly terrifying except perhaps for that hideous ballad in the background as Vernon rides his bike to school, but it's far more compelling in a (relatively) innocently creepy way than the slicker and more cynically formulaic eighties slasher flicks that followed. Like 'Plan 9' and 'Silent Night, Bloody Night' (both big in the 9/11 universe, and endlessly repeated since the broadcast rights must have been cheap... thank Jah I came of age before the infomercial era), it rightly belongs to the Cinema of Obsession, all the more convincingly when you're 16 and watching it for the fifth time at 2:45am.
There's something to be said for a teen revenge flick that could have plausibly been written and directed by a pimply adolescent.
Reading some of the other comments reminded me that this was also one of the films that made me realize that my early tastes in cult film weren't nearly so obscure as I thought. I remember being flabbergasted as a junior in high school circa 1979 to meet a fairly cute, well-adjusted girl from another school who had seen it three or four times