Slaughter High (1986)

Some time later (like, a few years, maybe), he lures the mean kids back to the now-abandoned high school for a fake reunion party... and from there, the picture turns into part haunted-house / part locked-in-a-mental-asylum / ALL-slasher flick, with the testosterone-driven jocks and the hateful bitch-cheerleader types dispatched every few minutes in a variety of ways, including impalement, an acid bath, a poisoning that results in an Alien-flavored stomach-exploding scene, the old reliable electrocute-the-copulating-couple scene, and a combination drop-the-jacked-up-car-on-the-guy-AND-slice-his-wrist-at-the-same-time trick (which has never been repeated since).

Finally, it's a showdown between Carol and Marty, who's been stalking her in a jester's suit (oh, his locker-room humiliation took place on April Fool's Day - which was his birthday too, the poor sap). She pushes Jester Boy through a window just as the clock strikes noon (when April Fool's Day is supposed to end) - but do you think he's really dead this time? Of course not. He chases her with a spear-like thing (a javelin, maybe?), and she ends up back in the girls' locker room, where everything started...Believe it or not, everything you expect does happen: It is Marty under the jester's mask, and he does kill Carol. No last-minute surprises here. Well, except that, after he kills Carol, Marty is confronted by the zombie-like ghosts of his victims, and suddenly, we're into Carnival of Souls / Dawn of the Dead territory...

But wait! There's more! None of this ever happened! It's all one big, horrible hallucination of Marty's - and (with his acid-burned head wrapped in bandages) he wakes up screaming in the hospital. A doctor, hearing his screams, enters his room...But wait! That's not really Marty under the sheets! The nurse leaning over the bed is Marty! And he jams a hypodermic needle into the doctor's face. It's possible this hallucination gave Marty the inspiration to invite them all back to the school, and something very similar to the hallucination did end up happening after Marty leaves the hospital.

SPOILED IT FOR YA!