Youths having fun at Myrtle Beach in 1963 RELEASED IN 1989 and directed by Zelda Barron, "Shag” is a teen drama about four girls freshly graduated from high school taking a vacation weekend at Myrtle Beach in 1963. There’s the senator’s daughter, Luanne (Page Hannah); the engaged or encaged Carson (Phoebe Cates); the wannabe Hollywood starlet Malaina (Bridget Fonda); and the thinks-she’s-still-fat-but-she’s-not Pudge (Annabeth Gish). The movie plays like a melding of “American Graffiti” (1973), “Dirty Dancing” (1987) and “Mystic Pizza” (1988). It’s far superior to the overrated “American…