WASHINGTON — FBI leaders and local law enforcement officials are studying shootings in schools to piece together trends and come up with ways to prevent future violence, officials said. The FBI on Wednesday hosted a daylong seminar at its headquarters for dozens of officials to discuss common warning signs of shooters, information sharing among law enforcement and response plans by schools. “We can’t allow ourselves to become numb to it,” Joshua Skule, the FBI’s executive assistant director for intelligence, said in an interview.