WASHING TON - The Supreme Court, with Justice Sandra Day O'Connor choosing the subject of unconstitutional sex bias to write her first opinion, says state-supported nursing schools cannot bar men from enrolling. Thursday's 5-4 decision is a victory for Joe Hogan, who broke a 97-year tradition by becoming the first man to enroll at the Mississippi University for Women. Justice O'Connor, the court's only female mem-• her, wrote for the majority that Hogan's exclusion from the university's nursing school violated the Constitution's guarantee of "equal protection" of the laws. "Rather than compensate for discriminatory barriers faced by women, MUW's policy of excluding males from admission to the school of nursing tends to perpetuate the stereotyped view of nursing as an exclusively woman's job," Justice O'Connor said. "By assuring that Mississippi allots more openings in its state-supported nursing schools to women than it does to men," she wrote, "MUW's admissions policy lends credibility to the old view that women, not men, should become nurses, and makes the assumption that nursing is a field for women a self-fulfilling prophecy." The university, located in Columbus, Miss., is the nation's only state-supported university for women. Hogan, a hospital nurse who lives with his wife in Columbus, wanted to attend the local university to obtain a bachelor's degree in nursing. His application to the university was rejected because he is a man. Mississippi Attorney General Bill Allain