The Sun incorrectly reported yesterday that former U.S. Representative Parren J. Mitchell became the first black graduate of the University of Maryland's School of Social Work, because of a court decision striking down a discriminatory admissions policy. In fact, Mr. Mitchell was the first black to attend graduate school at the University of Maryland at College Park, where he received a master's degree in sociology in 1952. The dean of the School of Social Work said yesterday that his school never had a policy of discrimination.