lazymum - that school in Big Little Lies is based on some RL examples. I know exactly what those parents are like, and the obnoxious behaviour of Reese Witherspoon's character in the pick-up line is so true to life. There are areas where the "public" schools are so exclusive they may as well be private. Local schools are funded by property tax, so if every house is worth over $1m, then effectively you get an enclave where only the children of millionaires are in the same school together.
And NK's "hippy" style is OK for that scenario. There are certain "types" within these enclaves. There's the hippy one, the business one, the fitness one etc. They are all variations on a theme. Hair and clothes are very expensively styled, houses are within a very small & exclusive area, they are all incredibly highly educated and often "liberal" but really very committed to preserving their own wealth.
These elite schools often have one or two kids who are on scholarships, or (like BLL) whose parents find a way to rent a shoe-box in the district etc. There's often lip service paid to how great it is to be so diverse, and a pretense of welcoming people (even fundraising so that more people can enjoy the experience of sending their kids to the school), but when something goes wrong, the "outsiders" are very quickly shunned.
David E. Kelly has a very good representation of this kind of community. It's what passes as being left-wing in the US, and you can see why people with less money find it objectionable. I suspect he is poking fun at himself and others who are part of these communities.