The new extracurricular entertainment at the Barrymore Theater wears its nostalgia on its sleeve as it riffles through a generation of yearbooks at Whatsis High. Ralph Keyes's book, ''Is There Life After High School?'' has inspired Jeffrey Kindley and Craig Carnelia to create a skipping continuity of song, anecdote, and vignette as nine young adults look back on their adolescence with mixed emotions. The result is a mildly diverting revue that celebrates with the winners and commiserates with the losers.
A sampling of composer-lyricist Carnelia's verses suggests the show's tone: ''The things I learned in high school . . . made me what I am today. . . . If I knew then what I know now. . . . If I was only half as good in high school as my second thoughts. . . . I don't want it now, I want it then. . . . There are thousands of trumpets in thousands of closets that never get played anymore. . . .'' One of the ex-high schoolers revealingly complains: ''I didn't grow up. I just grew older.''
Though none of it is particularly original, some of the fleeting biographical fragments are amusingly couched. From class picture to class reunion, the set pieces frame a past-to-present montage of middle-class, mostly advantaged white America.
The more pleasurable moments among the musical and very occasional dance numbers include a neat little ballroom pas de deux by David Patrick Kelly and Maureen Silliman, in which fantasy overtakes actuality; Sandy Faison and Miss Silliman's dueting of ''Fran and Janie''; and ''Thousands of Trumpets,'' a jolly miniproduction romp. Besides those already mentioned, the nine eager-to-please performers are Raymond Baker, Cynthia Carle, Alma Cuervo, Harry Groener, Philip Hoffman, and James Widdoes.
Bruce Coughlin and combo play the pleasant score pleasantly from an elevated bandstand that can descend or rise according to director Robert Nigro's requirements. The facility is part of John Lee Beatty's elaborate, multi-staired setting, lighted by Beverly Emmons. Carol Oditz's costumes are smart enough for the men but drab for the women. ''Is There Life After High School?'' might have made its modest mark Off Broadway. It doesn't pass the Broadway test.