NASHVILLE, Tenn. — A Tennessee police officer tried to prevent the arrests that would embroil his department in a national furor over policing in schools, but his colleagues and supervisors refused to change course.They insisted on arresting children as young as 9 years old at their elementary school and took them away— two in handcuffs — in view of waiting parents to a juvenile detention center as the school day came to an end.What followed in Murfreesboro, about 30 miles southeast of Nashville, was an …