All Los Angeles Unified School District schools have been ordered closed due to an unspecified threat, school district officials said. Los Angeles Unified is the nation’s second-largest school district, with more than 900 campuses and 640,000 students. Ramon Cortines, the school system’s superintendent, said the threat was against students at multiple schools. “It was not to one school, two schools or three schools, it was many schools, not specifically identified, but there were many schools,” Cortines said. Teachers and support personnel also have been ordered to stay away from schools. Cortines said he intends to have every one of the district’s schools searched. He said that while it is not unusual for schools to receive threats, this one was broader than usual and comes at a time of heightened concern about safety.