The black community must assume responsibility for stopping black-on-black crime, a National Urban League official said at a daylong conference on crime at California State University, Los Angeles. "We know who violates the rules. . . . Our young people are floundering now and I think the adult community has let them down," said Garry Mendez, director of the Urban League's Administration of Justice. About 75 people attended the conference, which was dedicated to the memory of Antoine Thompson, 18, who was killed Sept. 12 at Fairfax High School. Thompson was shot to death by reputed gang members during an argument over a hallway telephone.