I don't think you can categorize Massacre at Central High as a horror movie, although plenty of people die in it. There's no mystery as to who is doing the killing, and the entire film has a kind of "afterschool special" feel to it, albeit with a lot of gratuitous nudity. I think the movie's ultimate point is to deliver some kind of message about bullying, and fascism, and complacency, but the message gets a little muddy amongst the gratuitous nudity and violence.
That said, it's definitely a unique take on the high school horror story, even if most of the so-called high schoolers look like they've been legally able to drink for a decade or so, (including Robert Carradine who would still be playing a teenager eight years later in Revenge of the Nerds). Maybe that's why, until the very end, there are no "adults" to be found at Central High, or anywhere near it. It would be too hard to tell them all apart!
For me, the most the most entertaining fact about this movie is that the Italian version was called Sexy Jeans and included several edited-in scenes from porno movies. I'm dying to see how THAT worked.
I watched Massacre at Central High on the Criterion Channel.