From SFGate:
A girl was taken to a hospital after a 30-year-old woman came to her school and punched her, police said Monday.
The woman showed up at the 14-year-old girl’s Bernal Heights school at 11 a.m. on Friday and began arguing with her before the altercation turned physical, according to the San Francisco Police Department.
The woman then allegedly pushed the girl to the ground and punched her. The altercation was broken up by school staff and the woman fled in an unknown direction, police said.
IMAGE: Paul Revere School, by Telstar Logistics
Bernalwood 
How did a person from the outside gain access to a student inside without the staff knowing and monitoring the situation?
I guess the same way a kindergartner could just leave and go home undetected.
Boom!
Speculating here, but a mother or relative accusing a student of bullying her daughter/ sister? Or just a random crazy, I guess.