St. Mary’s High School will have a new principal when students return on Sept. 5. Mark DiFabio recently began work as assistant head of school/principal at the sixth- through 12th-grade Catholic coeducational school on Tremont Street. He replaced Carl DiMaiti, who moved to a new position as special assistant to the head of school, focusing primarily on enrollment and advancement. DiFabio, an Exeter, N.H., resident, previously worked at St. John’s Preparatory School in Danvers as assistant principal for freshmen and sophomores since 2010, and as a religious studies teacher for three years before that. Prior to St. John’s, he taught theology for eight years at Cheverus High School, a Jesuit school in Portland, Maine, where he also served as assistant athletic director. He also taught and developed undergraduate and graduate courses at St. Joseph’s College in Maine.