The door at the high school here is swinging open again. Ira Thomas, the school’s fourth principal in five years, has given notice that he is leaving. His last day was June 30. Thomas, 52, submitted his letter of resignation to Superintendent Barbara Wheeler, saying it was a “family decision.” Thomas, who grew up on the lower Eastern Shore, his wife and their daughters, now ages 6 and 7, moved here last July after spending three years in Massachusetts, where Thomas had been the principal of a 400-student high school. Thomas said he and his family would most likely move out of the area, and sooner rather than later as a new school year for his daughters will start in August.