On its official web site, the National Education Association is warning people to think twice before buying any school supplies at Wal-Mart. As part of its new “Wake-Up Wal-Mart” campaign, the NEA points out that the nation’s largest discount department store chain continues to block its employees’ attempts to unionize, while collecting public funds that could otherwise be used for education. The NEA also sharply criticizes Wal-Mart’s corporate support of private-school voucher initiatives.