**_Modern version of Carmilla at an all-girls boarding school in the Northeast_** Rebecca (Sarah Bolger) becomes suspicious of a mysterious new student (Lily Cole) at her private prep school. Is it just teenage jealousy heightened by trauma over her dad’s death or does Ernessa have a dark secret? “The Moth Diaries” (2011) is a psychological youth drama that eventually throws in bits of fantasy or horror. The latter is low-key with enough ambiguity about what’s going on to (mis)lead some. I can’t say more because I don’t want to give anything away. I’ll just point out that “Carmilla” is…