He was “the shy kid in the corner,” a classmate told the Pittsburgh Post-Gazette. Hours after a startling and savage attack yesterday that left 21 students and a security guard wounded at a high school in Western Pennsylvania, that was the picture that began to emerge of Alex Hribal, 16, a sophomore at Franklin Regional High School. Armed with two 8-inch knives, he is accused of stabbing and slashing his way through a crowded hallway in an assault labeled “bizarre” by a prosecutor and his own lawyer. One friend said: “I’ve never seen any anger from him, ever. … He never seemed like someone who would do anything violent.” Deborah Robinson, director of the University of Pittsburgh’s school social worker, home and school visitor certificate program, said threat assessment is “a huge part of what it means to be a school social worker.” Obvious signs of trouble could include poor attendance, poor hygiene and an inability or reluctance to connect with other students, though students who display those traits aren’t necessarily bound for violence. Indeed, students who commit violent acts may show no warning signs. “It’s really unpredictable. You can have a lot of good resources and good things in place and violence can still happen,” she said.