A friendship group doesn't have to extend friendship to you if they don't want to, this "they're all out to get me" narrative is very odd.
I find these "school mums are all bitches" threads absolutely baffling tbh. There's a set of weird assumptions on them which I just can't relate to and which frankly I think are pretty unhealthy:
a) you have to be on intimate terms with all of the other parents in your child's class
b) other parents are not allowed pre-existing friendship groups
c) if you don't invite every parent to every social event going and stop and say hello and chat extensively to every parent at the "school gates" every day you are a bitch.
c) "the bitches" are ganging up on you because there was a coffee morning way back when which you weren't invited to.
It's all really paranoid.
I can't comment on this particular situation, maybe these people genuinely are horrible. But I've read so many of these threads over the years and usually the common theme is that they are started by (usually) women who think they have some sort of entitlement to instantly become friends with every other mother who has a child in their child's class.
It's just not realistic or fair to expect busy people to carve out a huge degree of time for every other child's parent and to help you create your social life for you.
If you're lucky, a small handful of your child's classmate's parents might become friends. But they're just like people you would meet in any other walk of life. Some you will like, some you won't and most will be somewhere in the middle. There's no special magic about "school gate mums" which means they owe you friendship.
Instead of tying yourself up in knots worrying about minor sleights, you're much better off focusing on having a wide and diverse social network and learning to be a bit thicker skinned about the whole thing. Sorry if that sounds harsh. I'm just tired of reading this stuff all the time.