I don't think she got arrested for slapping the child. She likely was arrested for slapping the child several times.
There is a point where her action crosses from somewhat reasonable into temporary insanity.
I don't know about the public schools CA, but the public schools in TX have zero tolerance for violence on their campuses.
I don't see the mother being allowed to walk had she slapped the other child once, or simply pushed the little darling?
Schools stand in loco parentis, and corporal punishment has all but been outlawed on the local level.
Corporal punishment doesn't apply, as the mother wasn't acting in an official capacity (usually when a school imposes corporal punishment, only the principal can administer it after a due process hearing, not summarily).
The mother wasn't acting in self defense, would might have been an affirmative defense.
I see no other option other a criminal charge against an adult pounding a child unrelated to them on school property.
I've seen parents in TX prosecuted for popping their own child, Sweet Sally or Lovely Larry on school property.
KISD Police don't allow any violence to go unnoticed.
Here's another school assault.
Spring has sprung, kids are getting restless.
Killeen ISD police charge student at Palo Alto with assault