my mother and father have both dealt with multiple situations where they were equally insulted/threatened and did not call the cops on the offender.
my mom had one of my brother's teachers come across a table at her because she got really fucking pissed at my mom in a meeting. because my mom called her out on unfair practices in her class that affected my brother. the woman literally came across the table. with her hands raised. the asst. principal had to restrain the woman. my mom didn't call the cops. she didn't file a report with the school board or any bullshit. she talked to the asst. principal and the principal after the meeting, got everything straightened out and the teacher was disciplined in-house according to the school's policies.
your moms choice.
but if the teacher was to go postal the following year, it'd be good to have something filed with the police is my own opinion.
anyone coming at me with the intention to cause physical harm in a setting like that should be dealt with to the full extent of the law. your mother chose to trust the school and essentially trust the teacher to behave...that is your mothers choice and thank god it worked out for her.