I thought I was friendly with a group of school mums, particularly one as we both had 2 children the same age. The older 2 left for senior school and her child won the school award for Top Achiever, it was the "top" award if you like, given every year and could be for academic/sporting/community effort or whatever. The bitchiness started and one day she was crying at the school gate. I was really supportive and she appreciated it, was very grateful etc.
Three years pass and our younger children are leaving school, my child got the award, he was a bit of an all rounder not the star in anything but passed 11plus, was musical, captain of the football team so he got it for his range of achievements. Next day I walking up to schoolgate, start smiling as I am approaching her, she doesn't see me and I hear her being horrible about my son, he doesn't deserve it, never liked him etc etc. I stood transfixed and she must have felt my eyes on her and she turned and looked at me. She tried to speak and I walked away.
With my younger 2 I would say hello and smile, congratulate someone on a new baby or new house or whatever but that was it.
Sorry that was a saga, ridiculous that it stills annoys me 30 years later, not that I think of it very often but on occasions like this it comes to mind and I seethe all over again.
Honestly make your friends elsewhere, school mums as acquaintances is the best in my experience.