The principal and the police' hands are tied when dealing with students, at least students in elementary and middle school. Once they are in high school, assaults can be treated as such. They're not going to arrest a first grader who hits a teacher.
I am a special education teacher, and yes, I've been hit and kicked by children with emotional disturbance, adhd and autism. Rarely, though and never with an injury. My wife has sustained many injuries, black eyes, concusion, dislocated shoulder, and severe bites.
My wife works with what are called "life skills" students, severe intellecual disability (formerly known as mental retardation) and Autism so severe that it is effectively ID. For those kids, injuries are part of the job. You can't get mad at them for not being able to control themselves, any more than you should get mad at a blind kid for not looking you in the eye.
I work with kids whose behaviors are disruptive in regular classes. I pull them out as need, and they are often angry and aggressive. I avoid injuries mainly by staying just out of reach. Escalataing and getting in their faces is a sure way to be punched, and that happens often to AP's, who don't listen to my advice. What might work with a typical child won't work with a student who has emotional disturbance.
I don't know the details of that first grade teacher who was struck. But common sense tells me that he or she put himself in striking distance. I've never seen a child chase a teacher to attack them.
Every time I meet a child with emotional disturbance, there is some adult who brought that disturbance on. In the old days, such a child would have simply been expelled, and become someone else's problem.
I'm rambling. I probably should start my own thread about this, for teachers and non-teachers to talk it out.