The federal Transportation Department says more than 550 commercial driving schools in the U.S. that train truckers and bus drivers must close after investigators found they employed unqualified instructors, failed to adequately test students and had other safety issues. The department announced its latest move to improve safety in the trucking industry on Wednesday. Unlike a previous action last fall to decertify up to 7,500 schools that included many defunct operations, this one focused on active schools it said had significant shortcomings. The department has been aggressively going after states that handed out commercial driver’s licenses to immigrants who shouldn’t have qualified for them ever since a fatal crash in Florida in August.