“ The burden on a school board is to desegregate an unconstitutional dual system at once. Green v. County School Board, 391 U.S. 430, 439, 88 S.Ct. 1689, 1694-1695, 20 L.Ed.2d 716 (1968) ; Alexander v. Holmes County Board of Education, 396 U.S. 19, 90 S.Ct. 29, 24 L.Ed.2d 41 (1969) . Since the District Court ordered the desegregation measures into effect, and since the petitioners did not object to their scope, the Court of Appeals should have permitted their implementation pending argument and decision of the appeal. Alexander v. ”