They are just people like you OP. People who know each other stop to talk, nothing wrong with this, so there will probably be a group who know each other through older siblings enthusiastically chatting after not talking for the Summer. It isn't a reflection on anyone. You'll find there are also plenty first timers like you.
I honestly think much of the cliques and drama is played out in the imaginations of more introverted people. And I agree with PPs who point out it is a little sexist, if three male colleagues went for lunch together people wouldn't be whining 'why are they leaving me out', they would just assume they are friends, and so what if they are. As for the 'I've nothing in common with these people' brigade, well if your kids are the same age, go to the same school and you live in the same area, well what more in common do you need! If people only talk to those who share their specific hobby or profession then they are very narrow minded indeed IMO.
I honestly believe that making an effort with school parents can only do 1 thing and that is benefit your children. Parents are much more likely to arrange playdates with a kid if they know the parent even just from polite chat about the weather. No one wants to leave their kid with someone who comes off as cold and unfriendly. My advice would be to make the effort to strike up a conversation if you see someone alone, even if you hate to do this. You've worked jobs before where this is part and parcel of it I'm sure so this is no different.
I very much played the game. I had little family support and a new born twins but I went to every single coffee morning, night out etc even when it really didn't suit me as I wanted to make contacts. I also offered to carpool at any opportunity for parties etc, most parents were delighted. There were a few times where I had a genuine emergency and everyone bent over backwards to help me and thats when it paid off. I also benefitted from meeting some wonderful people, we are not friends as such but they are a circle of acquaintances. Also, my eldest was 'that kid' the one that is loud and troublesome and everyone avoids. I was used to hostility from childcare and playgrounds and often caught parents looking me up and down trying to judge me cos of my child's antics. This is one of the reasons I bent over backwards to meet everyone because my son needed this. I invited every single child to my house in the first term, some didn't go, some returned the favour, some didn't and that was fine. I know my boy would never have been asked anywhere if I hadn't put myself out there, 'that kid is trouble' was replaced with, 'he seems like a handful but the Mum is really nice, what harm can 1 day do'.