OP - I had to reply because it happened to me too! I have 3 children and they went to a different pre-school group than most of the others in their primary year as we lived outside the catchment area at the time ( eligible to go to the school because of my being a member of the affiliated church). An absolute nightmare.
I won't go into all the dull details but I remained an outsider throughout the whole of my children's primary years. We moved out of London to Kent half way through their schooling to buy our first home but that only made things worse for me as most women had made their friendships already. I spent years just dropping my kids off and running away and then going last minute to collect them. I spent 10 years of my life dreading the school gate mums.- who often appeared ever so judgemental but why I have no idea. why.
I never spoke to anyone much and kept myself to myself as I am a shy and quiet person in some ways. My youngest daughter leaves primary school next month - the last of my kids to do so and i am now very happily free of all of this as i found it very stressful. It's like an exclusive club where you HAVE to bake cakes, talk only about your kids and Anne Sunmmers, join the PTA, help run the church fair and basically behave like a happy 1950's housewife. Despite being a housewife myself - I do have a brain and I found the whole thing bizarre considering no one does this at secondary- it really is a primary school phenomena. I avoided it all- that was my choice in some respects but if you do so, prepare to be alone for a long time and sort of almost ostracized.
I do hope you make friends and not follow my example!
The main issue I had really was none of my kids knowing anyone else before reception - pre-school is so important to make connections. Even my daughter complained that 4 of her friends all went to the same pre-school and the mums were in each other's pockets way before that first school year started.
I just gave up after a while and never found anyone to bond with from the schools. I suffer from chronic anxiety too and there were so many very confident women who might say "hello" but who never really included you or invited you out. I felt they were all in competition with
each other somehow; who was the wealthiest, the best dressed, had the biggest SUV, the best hair do, the most sexy boots, the most influential friends. it never seemed to be about empowering women but about who could impress the most or contribute the most in helping the school- almost like a power trip for some of them. I know it was Greater London- the 'burbs' as such but I hated it. We rented and that was just well- far too 'common' for them and they made their attitudes apparent.
In Kent, I could never break into the pact of women's already formed bonds. Everyone knew each other in a smallish town and many grew up here - a less wealthy area , but almost the opposite- very insular. I gave up in the end sadly.
I wish you luck OP in making it work for you and that you get a good group of friends around you. I do disagree with the other poster- there ARE cliques but different types and each school has it's own type.
I am so sad that my anxiety and lack of confidence stopped me from trying harder to make friends and some of it is my fault, yes But not all and it's very difficult to 'fit in' with certain groups of women I think.