Mr. Shepherd is a most engaging raconteur who transforms small stories of everyday life into tall tales of fantastic adventure. In his words, an after-school encounter between Ralphie and a neighborhood bully named Scot Farkus becomes a collision of Olympian deities. Mr. Shepherd himself reads the sometimes very funny voice-over commentaries on the screen action and, you may be sure, is responsible for the bits and pieces of 1940s trivia we see. These include the sight of Ralphie's younger brother so bundled up in a pre-miracle-fabric snowsuit that he can't even move his arms. Those were the days.

Mr. Clark, the director of ''Porky's'' and ''Tribute,'' does not have a light touch. However, his heavy touch is not quite the same thing as Mr. Shepherd's habit of finding humor through the exaggeration of language. The movie's big comic pieces tend only to be exceedingly busy. Though Mr. Billingsley, Mr. Gavin, Miss Dillon and the actress who plays Ralphie's school teacher (Tedde Moore) are all very able, they are less funny than actors in a television situation comedy that one has chosen to watch with the sound turned off.

Most of the period details are good and accurate. However, I'm not at all sure that the curriculum at the Warren G. Harding Public School, which Ralphie attends, would require that 9-year-olds read ''Silas Marner.''

''A Christmas Story,'' which has been rated PG (''parental guidance suggested''), contains some vulgar language, though not one particular word that Mr. Shepherd, talking on the soundtrack, describes as ''the queen-mother of dirty words.''

Home in Indiana A CHRISTMAS STORY, directed by Bob Clark; screenplay by Jean Shepherd and Leigh Brown and Bob Clark, based on the novel ''In God We Trust, All Others Pay Cash'' by Mr. Shepherd; director of photography, Reginald H. Morris; film editor, Stan Cole; music by Carl Ziffrer and Paul Zaza; produced by Rene Dupont and Mr. Clark; released by M-G-M/U.A. At the Guild, 50th Street between Madison and Fifth Avenues; Loews 83d Street, at Broadway; Greenwich Twin, Greenwich Avenue and 12th Street, and other theaters. Running time: 98 minutes. This film is rated PG. MotherMelinda Dillon The Old ManDarren McGavin RalphiePeter Billingsley RandyIan Petrella FlickScott Schwartz SchwartzR.D. Robb Miss ShieldsTedde Moore Grover DillYano Anaya Scot FarkusZack Ward Santa ClausJeff Gillen Ming the MercilessColin Fox Flash GordonPaul Hubbard