An agreement announced last week allowing local National Education Association unions to affiliate with the AFL-CIO signals that the NEA is willing to tighten ranks with the rest of the American labor movement in the face of formidable economic and political changes. Once, leaders of the 2.8 million-member teachers' union may have believe they had more to gain than lose from keeping publicly aloof from other unions, including the NEA's longtime rival, the American Federation of Teachers. But the time has waned.