OP i feel for you, havent had experience of this myself but knew a very unscrupulous friend of a friend of mine who did. Herself, her dp and children were all ostracised by this one super bitch who basically ran the school social scene. She constantly pushed and picked, snide comments - in jokes etc etc.
The final straw came when she commented quite nastily on my friends choice of clothes for her dc. My friend told her that she was sure that she didnt mean to sound so nasty but that she was an older generation than my friend and therefore couldnt be expected to understand why some of the children were dressed more trendily than others, cue a systematic onslaught of nastiness, sending to coventry etc etc.
Friend approached her, asked what the issue was - was told she was common and not the sort of person that "one wished to mix with". It all escalated, friend's children ignored at lunch time etc, anyway - friend lost all reason, went to police, said she was getting dodgy calls etc (dont think she was), said that this lady had threatened her (again untrue from what I hear) etc etc and basically got the police to go to her door and speak to her.
Friend told all the playground what had happened and the school, local cub scouts, girl guides, vicar/parson etc. Nasty lady was apparently apopleptic with rage but couldnt do or say anything as it would prove friend's point!
Nasty lady backed off, friend smiles sweetly and asked local vicar to give sunday service on forgiveness and accepting people with all sort of health issues, ie mental health.
Honestly would not mess with this lady! lol - it worked though!!