Based on Gayle Forman’s bestselling teen novel, “If I Stay” should certainly appeal to its target demo. It alternately seems engineered for tears, giggles and swoons, and it shouldn’t have an issue with making teen girls, go through all of those in turn. Chloë Grace Moretz’s Mia Hall is an identifiable, admirable heroine, who isn’t quite the outsider she was in “Carrie,” but she still carries that teenage worry of being set apart from her peers. Meanwhile, her love interest Adam (Jamie Blackley, “Snow White and the Huntsmen”) is the perfect amount of sweet and smoldering for the audience. If BOP magazine were still around, Blackley would be its coverboy, adorning the lockers of high schoolers everywhere.