More than 500 student athletes have signed a letter to the N.C.A.A., pressing the organization’s leaders to stop holding championship events in states considering bills that would bar transgender athletes — mainly women — from competing in sports divisions that match their gender identity. “You have been silent in the face of hateful legislation in states that are slated to host championships, even though those states are close to passing anti-transgender legislation,” said the letter, which was sent Wednesday and signed by 545 athletes from at least 80 universities.