Feel for you op similar thing happened to me.
dd1 went to preschool next to primary and 22 went onto the primary which thourght be nice as knew some parents to say hi to. There was couple nice ones but 2classes and one i liked was in other class so dident get speak to her much.
There was just 2mums who reguarly spoke to me 1 wasent keen on and everyone tried to avpoid her she was fine in small doses but always so negative other lady was lovely.
I made effort to try make some freinds with new people met 1 and we became good mates but our kids couldent stand each other which made meeting up stressful
There was 1 mum I vauguely knew as her son went to dds nursery she wored in school rception and arranged a couple reception mums socials but i had small baby and hubby who worked long hours so dident make it to any.
I did try and make conversations in playground.
Then in year 1 the classes changed compsition so again ohers were waiting outside differnt class and all that group seemed to form a clqiue which included another mum who practically lives same rd as me we walked school same way dd1 would say hi and she never said hi to me was really awquard and she had been in dds preschool so basically known me for 2years.
Then dd3 was born so once again in playground with smal baby and tantrumming toddler and felt lot more isolated in year 1 like billy no mates. There was another mums whos baby was 4weeks older than mine went round hers for coffee, chatted to her at toddler group.
she then had massive party and invited most of mums from dd1s year apart from me and overbearing mums.
said freind became close to her and asked why and she said she dident know me very wel then had to face all the fb pics ad tags so then immediatly deleted her.
The rest of mums in dd1s seemed to pair off into best buddies.
then there was the clique.
then the outsiders which was me, anoying negative mum, another lady who i liked but he used to turn up late to avoid the mums, single parent mum as few single parents in the year, porn star mum people were ghastly to her real shame as she was nice and her outfits were entertaining as was heads face when she turned up.
The clique constantly arranged days out with their dds thankfully my dd was oblivious to it all. said mate woud say how lovely they were but also they used to be really awful over teachers collections and bitch behind peoples backs its made me quite miserable.
I did join pta but they had no social events and many of the parents were juniors so rarly saw them in playground.
I went back in year 2 ter 1 tried to be positive but no change dd was struggling so we moved schools which thought be really hard.
But each class had parent rep.
the school was small.
knew 2parents from sw group.
everyone was freindly and all diffrent groups mixed.
The pta bee freindly
alpha mums been nice always stop for chat.
There was one group of 3mums who initailly dident speak but made effort talk to them they now speak to me.
Lsst yera was mixed year 1/2class so know all the mums from year bebelow dds year but now shes year 3 know all the mums from year 3 and as its infant plaground all the new reception mums been very chatty and welcoming too.
Its like i become new confident persons.
dds had few birthday invites and gone back peoples houses for tea need to have more people over soon.
I havent made any freinds and havent really socialised with the much outside of school other than schools events and went running with few of them once. But feel less lonly and hopefully in time might make some connections espcially with dd2 and dd3 when they start.
I do think some years worse than others.
im sahm with 3 and its hard to make new freinds as adult.
most mus here meet at antenatal nct and ates ever since or meet through schools.
I do have other freinds but they live a distance and dont have kids.
keep trying focus on reception and 4mums you went out with,
be polite, confident but aloof.
maybe throw the best kids party ever
it does sound like one mum has a problem and the rest follow like sheep.
hope it improves for you.