My sister nannies a family where the father is shy and retiring, the mother has straight out Aspergers, all the four kids have aspergers, and the 7 year old eldest is so bad he's tipping into the Autism range: the violent Autism range. But the mother cannot see it, or won't see it, or can't cope with seeing that her eldest son, is at fault for beating on other kids, violently slapping or punching adults, screaming tantrums and general rudeness. She can only see the other side of the coin, which is also true: her son is feeling awful, and tired, and put upon, and not coping. But she's so busy trying to soothe her eldest, she's not seeing or addressing the real damage he's doing to his younger siblings, other people's children, or the nasty behaviour he has towards adults that's just not going to be tolerated when he's older.
My sister is privileged in that the parents love and trust her, she's widely sought and generously paid. It helps a lot that the mother works from home so my sister is theoretically overseen, even if she's mostly not overlooked. They trust her disciplining methods, which include a loose wrist lock to lead him to a time-out room, the one pulled slap she delivered one of the times he punched her, and her flicking him with water (which he hates) when he's violent. When out she's had to bodily pick him up and carry him away from situations, which she won't be able to physically do much longer.
Teachers are much more hampered, I would imagine, by tight disciplinary rules and consciousness of paedophilia charges if they're on their own with a child. Maybe they can't trust a violently distraught child alone in Time-out, but can't spare two teachers to monitor her, or one teacher with a door open and a child determined to escape. Maybe if they have a struggling child in a temper tantrum they can't constrain her without either allowing the child to injure them, or choosing restraint holds that tip over into the "child-abuse" or "illegal physical discipline" range. It's true physical discipline of children used to be too harsh and very abusive. But we're in the stage where we're overcompensating for that by not letting teachers physically discipline out of control children, and ALSO not giving them the resources to give them considered, psychological tools to handle problems in a structured way. So turning to the police it is.
I was arrested and charged for shoplifting as a tween, and for my punishment was called back in and given a stern talking to about what was acceptable and not acceptable, from a senior officer in a room, and let off with a warning. It was effective in my case. Obviously I would prefer really well resourced schools rather than teachers having to resort to police. But the police need to be there as a last resort. Finally: funding schools at a state level REALLY helps equalise school resources.