It is absolutely shocking and I feel for you and your DD. I have over the years known a number of parents who felt dissatisfied that the less selective 4/7 to 16/18 schools failed to prepare their children for the selective schools at 11+, you could assume an implicit aim of hanging on to their pupils right through, or more charitably that their school was one that aimed to avoid that sort of pressure, and if parents had chosen that ethos in the first place then it is fair enough to expect them to live with it, I am guessing from what you say you would not /did not have a problem with that. Traditionally Claremont Fan would have been one of those schools.
At the same time I have also known parents who had DCs in the preps for the more selective schools moan when they were told, sometimes after the 11+, their DCs were not going to be offered places in the senior schools. LEH used to make all their pupils sit the entrance exam so that although they had been made aware it was a possibility,it still came as a shock. As a result of parent dissatisfaction they now I believe have done away with the requirement to sit 11+ and pupils who are not going to be offered a place know well in advance, and can't really complain, they knew from the start a senior school was not guaranteed.
The problem is that you and your DD seem to have been hit with a double whammy, a prep school that didn't prepare them and / or didn't warn you that the Senior School was becoming more selective and might not be a possibility. Of course they will say that they had brighter applicants and it is true that some pupils might simply not have been bright enough to get into the increasingly selective senior school (and should have been warned ) but 20% were not bright enough to get to Claremont Fan? From it's Prep School? it sounds very much as if the preparation and / or selection process was at least part of the problem? It would be interesting to see what proportion don't get into the Senior School from St George's Junior School, another school that decided to ditch some of it's Junior School pupils in the relentless quest to become more academically selective but they at least They did / do it in full consultation with parents.
It would also be interesting to know how they designed their selection process, the most selective schools, like LEH, are looking for evidence of ability and personal qualities, not preparation, so it isn't first past the post. From having been on the inside of a successful Prep sometimes they surprise the Preps and offer places to pupils who would not come that far up in a race of attainment because though they are able they are not accurate, but have particular areas of talent. Creativity, lateral thinking etc are valued as much as accuracy.
I think Teddtess is wrong therefore, these schools are all looking for pupils with certain levels of ability who are right for their school. No amount of prepping is going to make a square peg fit a round hole. The added value of Prep Schools should be that they prepare children to pass the exam for the school that is right for them, and know the schools well enough to give sound advice on that. You obviously were let down.
I would now make very clear to the Governors of both Prep and Senior School how you feel about the level of communication, preparation etc. and how poorly it compares with practise at other schools, like St George's, LEH , you may know other parents who can brief you on the process in other schools, At least you can hope they can address the issues for the next set of parents.
It seems as though the schools that traditionally met the needs of those that did not do well in the 11+ for whatever reason are a shrinking breed. Are you in reach of St Catherine's? I am hearing of a lot of their offers being rejected. Worth getting in touch with all the less selective schools, including Surbitons which increased its year group. What has happened this year may be the result of the able pupils applying to more schools so places may still keep coming up, waiting lists exhausted even.