EVANSTON, Ill. — Northwestern University's football players will cast ballots Friday on whether to form the nation's first union for college athletes — a potentially landmark vote that will be kept sealed for months and possibly years. The National Labor Relations Board said Thursday it will hear an appeal by Northwestern challenging the decision from a regional NLRB director who ruled the players are university employees and thus have the right to unionize. The board said the ballots will be impounded at least until it issues its decision later this year.