A Ukiah High School security guard, Julian Williams, 39, was arrested Wednesday on suspicion of assaulting a 15-year-old student in a locked classroom Tuesday. Police said the student was not injured. Williams was booked into Mendocino County Jail on suspicion of child abuse and battery on school grounds. The Ukiah Unified School District, which refers to security guards as “campus supervisors,” placed Williams on administrative leave and stated it prohibits physical aggression toward students.