Because of an ongoing decline in student enrollment, Cathedral of the Holy Cross Grammar School will close at the end of the school year, said Terrence Donilon, a spokesman for the Catholic Archdiocese of Boston. The Rev. Kevin O’Leary, rector of the parish in the South End, made the decision to consolidate Cathedral Grammar into other Boston-area Catholic schools. An open house will be held at the school Feb. 6 from 6 to 7:30 p.m. to give parents the opportunity to learn more about each of the Catholic schools open to their children.