I’m sorry you’re having a tough time. I’ve been in similar situations to you, also left out of a year group What’s App. I’m older now and no longer give a shit.
There are always weird dynamics at play within class groups of parents. For the majority, these friendships are transient ones, with nothing in common other than happening to have a child in the same year.
You’ll get, for example, an Emma, who is social climbing and rushes to answer and be helpful to Kate or Jo.
You’ll have Claire, the PTA mum who generally knows everything about the school. She will either be the oracle or will hang back sometimes with answers for fear of looking weird. She might answer Alex’s question, because they go waaaay back.
Sarah, the badly organised mum, will asked questions, she will get answers because people know they can use her for emergency child care and want to keep in.
Amy overthinks things so she posts all the time questioning things, which annoys some people but a few others know her better and don’t mind helping her out.
Mel is everyone’s friend, but is mega busy and if she doesn’t reply right away because she’s only glanced at the message, she forgets. But if she sees a message from Kate, Emma or Amy she will reply as their sons play football together and they might prompt her to remember when she sees them.
Jess is menopausal so she also forgets if she doesn’t know the answer right away.
Wendy hates everyone so never replies.
None of this you would know on the surface. These dynamics are complex and can drive us mad or to upset trying to understand them.
Dont use other parents as secretaries. A lot of info you can find out yourself.
Dont guess or overthink other people’s motives. You are worth more than the unnecessary worry that brings you.