I am a grandparent carer so have done this all before. I speak to those who speak to me, but have no time for the silly women who exclude others to make themselves feel important. Once one clique falls apart (and they do) another one takes its place, such is life.
I don't generally bother until it affects the children and when it began to, I moved my youngest gs. One clique of parents had taken over after school clubs and certain children were being left out of everything with preference for their mates' children - nothing to do with ability. There were other serious issues in the school, but as these parents were on the governors, it was clear it was easier to move rather than challenge.
I would not consider myself over-sensitive op, but it really depends on how much you can take. The main issue is does it affect your dd?
There were a group of 3/4 mums in my eldest ds's year and they were the rudest, most ignorant people I have ever met. Two would speak to you if they were alone, but in the group, forget it. I thought of them as the witches of eastwick.
I understand what you say about assemblies, plays, etc. The front few rows always had coats on them no matter how early you would arrive. Once I attended a praise assembly, sat down and the queen bee mum arrived and saw her friends sat by me, walked up to me and stared at me indicating I should move. No chance. She stormed off to sit alone.
His new school is so, so much nicer. I just drop off and do not go into the playground and he is older now. Staff are kind and do their best, big mix of kids backgrounds, some problems, but nowhere is perfect. The main thing is that he has grown in confidence and is so much happier.