It’s a practice that just won’t die. Study after study, researcher after researcher, has made the same point: Holding students back when they are not achieving at grade level does not help them academically. Still, the idea resonates with the public. And outgoing Mayor Richard M. Daley garnered praise for instituting a ban on social promotion in 1996. Now, like an aging, punch-drunk prizefighter who just won’t give up and leave the ring, the district’s promotion policy remains alive, if not well.