The state’s highest court has ruled Lowell school officials can’t fire a teacher who failed two English fluency tests. The Massachusetts Supreme Judicial Court said a lower court could not overturn an arbitrator’s decision in favor of the teacher, Phanna Kem Robishaw. Robishaw began teaching in the Lowell public schools in 1992 after fleeing the Khmer Rouge regime. She was one of five teachers at her school of Cambodian birth, where nearly half the student population was Cambodian.... ....Robishaw had four state teaching licenses but failed the English tests in part, she said, because she was suffering from post-traumatic stress.