MARBLEHEAD – Superintendent of Schools Paul Dulac stepped down as superintendent Thursday – nearly four months ahead of schedule – for medical reasons.Dulac discussed the decision with the School Committee March 3 and announced it to the faculty and staff at a professional development event Thursday morning at Marblehead High. He received a standing ovation at the conclusion of his remarks, in which he expressed his faith in the staff and faculty.On his recommendation, the School Committee has unanimously named School Business Manager Brian Salzer to serve as acting superintendent, starting today. Salzer will continue until the next superintendent of schools, Greg Maass, begins his duties July 1.Dulac will be on medical leave as of today through April 1, working a half-day schedule as “a reference and resource” to Salzer, according to School Committee Chairman EuRim Chun. From April 1 until his contract officially ends June 30 he will use vacation days.Maass is winding down his duties as superintendent in Green Bay, Wis., but plans to be in Marblehead at the end of this month to discuss the transition and choose the next principal of the Marblehead Veterans Middle School.The School Committee approved the arrangement after Dulac provided them with a copy of a letter from his doctor. Dulac said Thursday that he was not ill, but his doctor told him he returned to work too soon after undergoing heart surgery last spring and he needed time off to fully recover.In a press release, Chun said the School Committee “fully supports” Dulac’s decision to step down but accepted it “with great sadness.””Over the past four years, the Marblehead School district thrived under his leadership,” Chun said. “Dr. Dulac leaves a legacy that (fostered) critical thinking and creative problem-solving skills in our students. Dr. Dulac brought the district forward to better provide “?all Marblehead students with the 21st century skills?to compete on a global scale and to live better lives.”The decision leaves Salzer to oversee the final preparation of the Fiscal 2012 school budget, completion of the Marblehead Village School project and the second attempt to build a new Glover School.