Discipline had become a "dirty word" in many schools and children had become "the new untouchables".
Mr McKelvie, the union's treasurer, said: "They are above and outside the law and there is nothing we can do about it. In my school they are able to negotiate and write their own timetable."
If there was only one such destructive pupil in school of more than a thousand pupils, it was still one too many, he said. The conference backed his appeal.