A recent vicious attack on a 15-year-old boy by 11 classmates was only the tip of the iceberg as many victims rarely complained, educators and social workers have warned. Sammy Chiu Wai-sang, associate professor of social work at Baptist University, said reports had found violence was prevalent in schools. 'Such cases are under-reported because the violent act does not always take place in front of teachers. Face slapping in the changing room and pushing and kicking in the corridor are only two examples,' he said. Concerns about school violence have increased after a video of a Form Five student being viciously beaten by schoolmates in a classroom at De La Salle Secondary School, Sheung Shui, was put online and sent to newspapers last week. The 11 students, who posted bail earlier this month, have to report back to police today. Professor Chiu urged the government to acknowledge the problem and devote more resources to schools. He said schools should strengthen their relationship with social services, who had experience in dealing with troubled youths. Chan Kwok-kuen, one of the directors of the Hong Kong Professional Teachers' Union, said teachers should provide a model for students to cope with conflict. 'Teachers who like to shout or pound their fist on the desk every time there are problems in the classroom will make their students think that violence can solve a problem,' he said. But the Education and Manpower Bureau said student violence was not a serious problem. Annual surveys indicated a drop in the number of cases from 527 in 2001 to 427 last year. Social worker Fermi Wong Wai-fun warned teachers not to underestimate verbal threats, which could be an early indication of future violence. 'It usually starts off as one student's attempt to verbally bully or manipulate another person,' she said. 'If nobody stops the student, the problem will escalate into physical violence. Verbal threats are also a form of violence. 'Students who were repeatedly threatened with being beaten up have told me they wanted the threat to be addressed soon so their nightmare would end.'