MrsCollinssettled - of course the school wouldn't advertise it. The school has no control over it. The whole point is it is set up by parents for parents. Schools do not particularly like these sites, as if not properly moderated by the person who set them up, they can become venues for bullying and malicious comments about children, parents, teachers and the school, and parents need to be reminded of the laws of libel, and the inappropriateness of bullying and malicious gossip. If run properly, they are just a slightly dull but useful place to find out who else might be going on a trip and be able to give a lift share, get reminders of homework, translate garbled messages passed on by children, repatriate lost property, ask if anyone has a spare whatever-annoying-last-minute-thing-the-child-needs-to-take-into-school-tomorrow, etc, etc. You could always try searching your child's school on Facebook or similar apps to see if anything comes up. The notion that people should be coming to you and inviting you is a bit far fetched. It's not hard to find out for yourself if such a thing exists.
How does your dd get to and from school? Does she walk by herself, or have a childminder to take her (who could find things out for you), or do you do it? Do you not live near anybody else with children at the school? Do you not have contact details for your children's school friends' parents? You make yourself sound unusually isolated - far more so than the average full time working parent.
Finally, how have you approached the school about late notice? Headteachers are only human and they tend to feel more able to offer fulsome apologies to parents who come across as polite, reasoned and empathetic themselves, rather than, eg, angry and self-righteous. And it requires the headteacher to change the communication culture of the school - which they are more likely to do if they are not all riled up themselves and passingmon angry, unconstructive messages to staff so that everyone feels threatened and defensive.
As I've said, I have no experience of schools with a massive communication problem, so it is hard for me to imagine a school which genuinely does give virtually no notice for anything and I can't think of a school having a good excuse for that.