I was in exactly that same position last year at a tiny village school. It was horrendous, particularly as she seemed utterly unashamed at the time. What made it even worse was that she had apparently been my best friend for two years.
I kept my dignity, have blanked her at all times ever since, apart from one short text exchange in which in I carefully and in a controlled fashion, left her in no doubt as to what I thought of her behaviour. My 6 year old DD and hers are in the same class. My DD knows that my 'friend' and I had a quarrel and I wasn't about to forgive her and she took that at face value.
A few of my friends knew the situation and in an odd move, she later shared it with a friend of hers and within minutes it was round the playground. She seemed surprised that people weren't very supportive of her actions and I felt much better when it was properly out in the open. Most people were most impressed I hadn't put a brick through her window.
This was September and whilst I had a few tense months whilst she tried to brazen it out in the playground, the tide eventually turned and she has kept a very low profile ever since. My DH and I are reconciling, her marriage broke up anyway, and funnily enough, whilst people round here are generally very helpful, one seems keen to offer their DH's up for any odd jobs etc.
I agree with others; treat her as though she doesn't exist. And I read something recently that struck a chord; whilst you are nursing great hurt at injustices done to you, the other person generally doesn't care - that's presumably why they behaved that way in the first place. So be kind to yourself and try to drop that burden of care as soon as possible. Living well is the best revenge.