That woman is a disgrace and if, instead of keeping her head down, and facing up to the consequences of what she did (i.e. quietly accepting whatever punishment she gets), she is whipping up a frenzy of hateful gossip then she is a totally arrogant and irresponsible bitch.
I'd keep quiet about confronting her, or even getting involved in justifying yourself to others unless anyone directly questions you.
However, if this does end up in court, I'd make damn sure that the local newspaper was advised there might be a juicy story there for them. An over the limit mother in a 4x4 (in particluar) on the school ruhn - they'll be bound to headline that one.
And then what will the callous cow say to her ignorant, nothing-better-to-do-than-tittle-tattle hangers-on ???