With its hipster irony, butch posing, and blue-collar affectation, the postfeminist fad for women’s roller derby doesn’t seem to promise much more onscreen than a goofy exploitation quickie. But this comedy about a Texas high schooler (Ellen Page) who reinvents herself as “Babe Ruthless” and becomes the star of an Austin league is a surprisingly credible coming-of-age story. Drew Barrymore, making her directing debut, never allows her eccentric bad girls (among them Kristen Wiig, Juliette Lewis, and Barrymore herself) to stray into caricature, and the heroine’s conflict with her conventional parents (Daniel Stern, Marcia Gay Harden) is warm and credibly handled. (Watching Page and Harden work together is a particular treat.) The skaters may pride themselves on being as brutal as the guys, but Barrymore’s accomplishment lies in putting this across with such a gentle touch. Shauna Cross wrote the script, adapting her own novel. PG-13, 111 min.