They didn’t call for an all-out boycott, but presidents of dozens of liberal arts colleges signaled their intent Tuesday to stop cooperating with U.S. News & World Report’s college rankings. What that means exactly is not spelled out. Because most college presidents must answer to trustees, faculty and other constituents, “the decision to participate or withdraw from the U.S. News rankings rests with the individual institutions,” says a statement released Tuesday by the Annapolis Group, whose members include 121 liberal arts colleges.