single parent here. I went through very similar and drove myself a bit potty with it making myself really upset. I don't get how some parents can be so downright rude! It was things like a kid and their mum would pop over for a playdate, we'd have a great chat, then the next day she acted like she didn't know me when I saw her! Another one looked the other way in the corridor when I walked past etc etc. Really odd (and I don't think there's anything particularly odd about me or my kid!!).
I just put it down to cliquey, snobby behaviour a bit like being back at school again.
The only way to deal with it is to completely mentally detach from any expectation that they will be friendly, stop trying to be friendly with them, if someone talks then it's a bonus but don't initiate any conversations unless they arise naturally, and even then don't expect anything more than a quick chat.
Act as if they are strangers and release your expectations for anything else, mentally detach and focus on the rest of your life outside of these people (family, friends, clubs etc).
It's unfortunate that in this day and age people act as if they are in the playground themselves still! There's so much snobbery, classism, cliquey, alpha type behaviour still.
I have come to the conclusion that either people are very busy with their own lives, stuck up or repressed. I think it's a british thing in particular, a cultural issue that runs deep in our society. That's how I've accepted it.
Just know you're not a bad person and it's more about them than you - and get on with your own life as if they never existed!
Not always easy but primary school goes quickly and it won't last forever.