Twelve white youths charged with attacking a black man and chasing him to his death Dec. 20 in New York's Howard Beach section will stand trial in two groups, Justice Thomas A. Demakos ruled. Four teen-agers charged with murder or manslaughter in the death of Michael Griffith will be tried together beginning Sept. 8, he said. Seven others charged with crimes from attempted murder to inciting a riot will be tried at an undetermined date. The 12th is cooperating with prosecutors and is expected to plead guilty to a reduced charge of assault. Griffith, 23, was killed by a car while fleeing a gang of white youths.