Teachers physically hurting children is completely illegal and they could be imprisoned for it.
However there is a bit a of a loop hole with things like ARS 13-401 Justification; use of physical force, section 1 (I think that is an Arizona legaslation), which says teachers 'may use reasonable and appropriate physical force' to maintain discipline, but hitting kids would not fall under that IMO.
You should complain to the police if this really is the case.