Travis Marshall has had enough of Frankie Murphy's bragging about Philadelphia, where he used to live, and about the girls he's kissed. Most of the other sixth graders think Frankie's the coolest thing that's ever come to Harpersville, but Travis wants to call his bluff. He bets his beloved Oakland A's jacket that Frankie can't kiss all the girls in class before the end of school, in five weeks. Starting with ``weird'' Penelope Finchester (who makes and distributes buttons to save endangered species), Frankie rises to the challenge, and soon there's only one girl he hasn't kissed—Travis's best friend, Annie Davis. But by then, Travis has realized that losing his jacket is not the worst thing about the bet: what's really wrong is tricking and taking advantage of the girls. Still, they have the last word: on promotion day, Penelope hands out buttons to the entire class. ``I'VE BEEN A SIXTH GRADE JERK,'' read the boys', while the girls' proclaim, ``FRANKIE MURPHY IS THE WORLD'S WORST KISSER!'' Believable, endearing characters, snappy dialogue, and a good story make this latest from Guthrie (The Witch Who Lives down the Hall, 1985) an authentic, funny slice of pre-junior-high life. (Fiction. 9-12)