The charm of Korman's latest ""Bruno & Boots"" story lies in his ability to make the total chaos of slapstick comedy utterly believable. The formula is readable and easy: the movie being made at the boys' boarding school features a teen-age idol who has students at the girls' school across the way gaga. Bruno, determined to be an ""extra,"" so irritates the director that he is forbidden access to the set. He and Boots befriend the star, who leads a lonely life protected from the trappings of ordinary teenhood. Even readers who haven't read Korman's other books will know it'll all work out, but the grand finale at a remote camping spot is funny all the same. Clever throughout.