The head of the National Education Association opened the largest school union's annual convention yesterday with a call for public school teachers and employees to mobilize politically to help defeat President Bush this fall. "I know that if we put forth our best effort, we are going to win," Reg Weaver told a cheering audience in a 30-minute speech in which he criticized Mr. Bush and Education Secretary Rod Paige. "Our 2.7 million members can be the 'X-factor' in this election. We and our pro-public-education allies can and will make a decisive difference," he said. The convention votes tomorrow on...