A bitter old history teacher at a wild English high school (Judi Dench) befriends an attractive young colleague who’s just arrived (Cate Blanchett), only to discover she’s having sex with a 15-year-old student. The skillful Patrick Marber (Closer) adapted this gripping drama from a novel by Zoe Heller, and it’s both literate and urgently plotted, with a voice-over from Dench that cuts like broken glass. Her character is sly, controlling, desperately lonely, and capable of anything, and when Blanchett’s secret gets out, a proper chamber drama explodes into something much more troubling. Richard Eyre (Iris) directed, and there are fine performances from Philip Davis as a teacher with a crush on the young woman and Bill Nighy as her unsuspecting husband. R, 91 min. a Century 12 and CineArts 6, Pipers Alley, River East 21.