HAIR HIGH — ** — Animated feature starring the voices of Dermot Mulroney, Sarah Silverman and others; not rated, probable R (violence, vulgarity, sex, gore, nudity, profanity).

As diverting as the best of Bill Plympton's "Plymptoons" can be, they're best taken in small doses. After 30 minutes or so, watching his signature animation — which is deliberately crude and shaky — becomes tiresome. And his latest film, "Hair High," is a 78-minute-long feature!

He also has a penchant for gross-out gags and other cheap humor, which has dragged down some of his longer works, especially the 1997 feature "I Married a Strange Person."

That's even more the case with "Hair High," which wastes a first-rate voice cast that includes some of Plympton's celebrity friends and fellow animators.

Sarah Silverman and Dermot Mulroney voice the characters of Cherri and Rod, popular high school students who become involved in a love triangle — one that ends badly for at least two of the teens.

And there's not a whole lot more to the plot than that.

Plympton uses the high school setting to send up period musicals, in particular "Grease," and the '60s "Beach Party" comedies that starred Annette Funicello and Frankie Avalon.

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He also indulges in crude, infantile spoofs of horror movies and sports dramas — the latter, with a lengthy crescendo featuring a game — and an orgy of sorts (which was seen in shorter form in the most recent "Spike & Mike" compilation).

His clear intention is to outrage, but he's done this sort of thing before, and better (in "Strange Person," as well as several of his shorts).

It's probably time for Plympton — who's been making films for nearly 30 years — to try to do something a little loftier.

"Hair High" is not rated but would probably receive an R for scenes of animated violence (exaggerated athletics, slapstick and killings), crude sexual humor and language (including profanity), as well as other vulgar humor, sex and other sexual contact (the aforementioned orgy), gore and animated nudity. Running time: 78 minutes.