LOS ANGELES (AP) _ Like air raid sirens of the Cold War, a special bell now alerts children at an elementary school that violence is just outside the building and sends them ducking for cover.

Students at Langdon Avenue School are trained to return to class when they hear what some parents call ``the shooting bell.'' It signals that gunshots have rung out in the North Hills neighborhood 20 miles northwest of downtown.

A truce among Hispanic gangs in the San Fernando Valley appears to be unraveling, driving up the number of shootings there.

``This area is terrible. There is much violence, so they must have this shooting bell,'' said parent Elsa Ceja. ``It's too bad for the children that it's part of what they learn at school.''

Children are taught on the first day of school that if they hear a long bell, they must immediately leave the playground or lunch area and head to their classrooms, where the doors are locked.

``It's really sad, but it's reality,'' said Nereida Johnson, a community liaison in the area for the Los Angeles Unified School District. ``For people who don't live in areas like that, it's hard to believe.''