“Monsters, Inc.” may be the most original and underrated film ever to come out of Pixar, an inventive spin on the professional bogeymen hiding in kids’ closets and the reasons they’re there in the first place. It’s their job, and a fully realized alternate universe is utterly dependent on their work.

“Monsters University” is a prequel that is far more conventional, not nearly as witty or clever as that original. The 3-D animation takes the art form to a new level, a few sentimental moments connect, and the climax is a hum-dinger. But with the bar set so high by the first film, this “Mike & Sully: The Fraternity Years” can’t help but feel like a letdown.

“University” takes us back to Mike Wazowski’s childhood, when the smallest, shrimpiest-voiced monster on the bus resolved, after one school field trip to Monsters, Inc., to become a “scarer.” It doesn’t matter many people tell him, then and later, that he’s not scary. He figures that just by wanting it and working harder for it, his dream will come true.

Years later, at Monsters University, Mike (voiced by Billy Crystal) rooms with Boggs (Steve Buscemi), a mousy chameleon trying to fit in, still years away from becoming Mike’s nemesis. The REAL enemy in college is frat-boy slacker James Sullivan (John Goodman).

Everything the tiny, one-eyed Mike must study and study for, Sully seems ordained to achieve simply by genetics. He’s bigger, louder. He’s a “legacy” scarer. His dad was a legend. He doesn’t study, doesn’t apply himself and gets on Mike’s nerves at every corner — UNTIL they’re both dropped from The Scare School and forced to work together with members of a fraternity of outcasts to win their way back in through The Scare Games.

So that’s our plot, a sort of “Animal House” / “Old School” / “Revenge of the Nerds” retread of every frat comedy to come down the pike, with a Hogwarts (supernatural school) twist. Sully steals a competing school’s mascot, fraternities bicker over the most desirable pledges and Mike and Sully find their way from hate to the friendship that we see later in “Monsters, Inc.”

Helen Mirren voices the dragon / scorpion-like dean-of-the-school, Alfred Molina’s a popular teacher and Nathan Fillion is the sneering leader of the Roar Omega Roar fraternity. Sully and Mike are stuck with the Oooze Kappa boys (“We’re OK!”).

It’s too long, the jokes aren’t as funny, but the sight gags are doozies. And the film’s gorgeous 3-D sheen shows just how far computer-generated animation has come since 2001.

But aside from the adorable live-action-plus-animation short attached to the beginning (“The Blue Umbrella” shows the hazards of two umbrellas meeting and finding love in the big city) and a real horror-movie tribute finale, this “University” comes up a few credits shy of a degree.