Billy Bob Thornton is certainly good at playing mean teachers. He easily filled the part of the malevolent professor of one-upmanship in the recent remake of “School for Scoundrels,” and now he’s equally hissable as a Gestapo-style small-town middle-school phys ed instructor in “Mr. Woodcock.” But he’s about all the movie has to offer: though it’s better than the sleazy title might suggest, this academic tale doesn’t earn a passing grade.

The best part of the picture is the flashback section showing the ruthless martinet Woodcock systematically tormenting young John Farley (Kyley Baldridge) and the rest of his gym class in their tiny Nebraska town. This is cruelly funny stuff, with Thornton’s nonchalant nastiness paying dividends.

Unfortunately, that material is just a sliver of the movie, most of which is set years later, when Farley (Seann William Scott), long away from the Midwest, has found success as a self-help author, his latest bestseller a tome titled “Letting Go,” about putting the past and its unhappy memories behind you to achieve calm and self-confidence. But when he gets an invitation from his hometown to come back for its annual Cornival and receive its biggest award, the Corn Cob Key, he drops out of his book tour to the consternation of his pushy agent Tracy (Amy Poehler) and flies back for a brief stay with his widowed mom Beverly (Susan Sarandon).

To his horror, John discovers that his mother is dating again—and of course her new boyfriend is Woodcock, the very person he’s had to “let go of” to achieve his sanity. And when Woodcock proposes to his mom and she accepts, he’s apoplectic, and tossing aside all the advice he’s made a fortune giving to others, he sets his mind on revealing the sort of creep the teacher is not just to Beverly, so she won’t marry him, but also to the whole town, so that it will reconsider giving the guy its teacher-of-the-year award. The script degenerates into a series of blundering attempts by John to unmask Woodcock, in which the coach repeatedly turns things around to humiliate his old student anew, and contests in which John struggles futilely to best the old man at last. He shows he’s no great judge of character when the person he enlists to help him is old classmate Nederman (Ethan Suplee), a nerdy fellow who’s obviously a few cans short of a six-pack. And, of course, Farley takes time for romance by reconnecting with Maggie (Melissa Sagemiller), a girl he once had a crush on from afar who’s now a teacher. Of course, his increasingly frantic efforts to destroy Woodcock nearly ruin his chances with her.

All this is pretty feeble stuff, especially since after “Scoundrels” it comes across as a case of “been there, done that” and doing it not quite as well this time around. Partially that’s the fault of the basic premise, since attempting to break up your mother and her boyfriend is a good deal creepier than competing over a girl (which was the premise of the earlier movie). But it’s also due to the fact that the comic bits that novice scripters Michael Carnes and John Gilbert have come up with are rather weak, and the ending they’ve devised not just implausible but dumb.

Apart from Thornton, the cast doesn’t help much, apart from Poehler’s sharp work as the process-obsessed agent who can’t even bother to read the book of the author she’s representing. Scott is bland, even compared to Thornton’s “Scoundrels” rival Jon Heder, as is Sagemiller, and Sarandon is curiously colorless (an understandable reaction to having so little to do but stand around looking uncomfortable—especially when she has to don a “Queen of the Corn” parade dress). With Suplee merely repeating his “My Name is Earl” shtick, the interplay between him and Scott doesn’t have much zip, either. A good deal of the blame for the flat performances has to be laid at the feet of Craig Gillespie, whose direction is flabby, but technically the movie’s okay, if the sort of thing that will lose nothing when shown on a TV screen.

“Woodcock” isn’t as gross as most Hollywood comedies today. It’s not offensive, just anemic. And even Coach Billy Bob can’t whip it into shape.