I would agree with OP and Mominatrix's early posts.
London day schools are struggling with a sky high parental aspirations, but SPGS is in a different league altogether, and from the outside there is no indication that the school are trying to change it.
Some years ago but DD was taking part in a holiday activity in North East London. At the end of the second day the instructor asked if DD was having problems with one girl who was clearly super competitive and over-bearing. I think he wanted to have a word with the parents but was seeking some evidence of the impact her behaviour was having on other children. I said that DD was OK because she was used to Paulinas, which triggered questions from the confused instructor about why such behaviours were deemed acceptable for pupils from any school. He was right.
The confidence is odd. Even women in late middle age seem to need to tell you they were at SPGS, so it is long-standing. Now coupled with super competitive London parenting, with a place at SPGS being seen as the ultimate goal, and it can become toxic. We certainly knew of some casualties.
The fundraising sounded a bit scary. I once heard a seriously affluent Kensington banker's wife boasting loudly about being invited to one of Clarissa's dinners. "They are only after your money" said another less affluent parent who had never been invited. "Yes I know. I think she must research us." Similar tales of being interviewed by other parents before you could be invited to take part in parents activities, and the poshest school fete in West London. There seemed to be a lot of us and them going on amongst the parents which appeared to extend to the girls. Plus some astonishing involvement in a child's day to day school life, and some pretty objectionable competition amongst mothers. I really did hear mothers write off their DDs friends as "not very bright". And of a girl who amassed a 100 pairs of shoes during year 7, as in her friendship group being bright was not good enough. No uniform policy and you had to be seen to be cool. (I preferred the G&L girls on the tube who were wonderfully creative about what they did with their unpromising uniform.)
One reason we turned down a place at SPS for our DS was after hearing a Colet mother describe one of DS' friends who had gone there aged 7 as not nearly as bright as her son, who apparently ranked third in the year. We did not want to be part of that. Westminster had a big advantage in that it ran like a boarding school so contact was via housemasters, and boys/parents had very little idea of how others were doing.
I would not put it all down to Clarissa. I suspect the governance structure may be partly to blame for the focus on high grades. (My observation is that Westminster, which is essentially led by representatives of the CoE rather than the City, have more leeway to recruit pupils who will add to school life.) Not least it will be responsible for the high fees, which has led more than one parent to opt for Putney High - simply because the difference did not seem worth the cost of a family holiday. (Even to attend the sixth from open day would have cost us about £150: Westminster and KCW did not charge - and we did not bother.) The change in catchment from upper-middle class journalists, doctors, MPs and academics to bankers and the international super rich will also have had an impact. There is some astonishing 11+ tutoring going on, the impact, like with Tiffin, is for some parents to decide there are other good options and that a less pressured childhood is more important.
That said, parents told me that when Clarissa started there was a distinct move towards a greater exam focus. She may claim now to be worried about this, but she almost certainly was part of the problem.
Still I am looking forward to the book. I hope it is as gossipy as John Rae's memoirs of Westminster School in the 70's, though I suspect it won't be.