NASHUA - A Nashua High School South headmaster whose military history was being investigated by the school district has resigned. Robert McDowell, the ninth-grade headmaster at the school, sent in a letter of resignation late Wednesday night, Superintendent Joseph Giuliano said Thursday. Giuliano and Principal Tim Kelley had been investigating McDowell’s military past since late last week, when Kelley received a copy of a January 2002 letter signed by McDowell to the Secretary of the Navy. In the letter, McDowell, a Portsmouth resident, requested an “other than honorable” discharge and announced his resignation from the Navy, Kelley said. In the...