“ Where a preliminary injunction has issued to vindicate constitutional rights, the presumption in favor of the District Court's action applies with particular force. The Court of Appeals did not suggest that the District Court abused its discretion. On the contrary, the Court of Appeals expressly stated that the District Court's findings of fact 'represent a painstaking analysis of the evidence presented. They establish a racial imbalance in certain named schools. ”

“ County School Board, 377 U.S. 218, 234, 84 S.Ct. 1226, 12 L.Ed.2d 256 (1964) . 'The burden on a school board today is to come forward with a plan that promises realistically to work, and promises realistically to work now.' Green v. ”