I think it is common for independent schools to 'weed out' at the end of each phase, if they have high academic standards. It is unpleasant, but I think that as long as the possibility is made clear on joining the school and advance warning is given, rather than just a shock booting out, then this is the price people have to pay for highly selective education.
My DC go to a Prep which makes clear that moving from the pre-prep to Prep at 7, and to the Senior School at 11 is usual but not guaranteed. At the end of Yr 1 and Yrs 3 and 4 parents are told if their children are 'on track' for the next stage. If they are not on track, the idea is that parents start considering elsewhere for the next phase .....they have a year for those in Year 1 and up to 3 years to consider alternatives if in Year 3. The idea is that no one is suddenly told with less than 18 months to go, that their future in the school is uncertain. There is time to consider alternatives. Of course, no one likes being in this position and usually it is only about 5 per year if that, that are. For those not getting into the senior school, they are warned at the end of Year 3, told more clearly in Year 4 (if there hasn't been improvement) and by the end of Year 5 will be getting a very clear message. Those who are not guaranteed automatic entry to the senior school can still sit the entrance exam with those from outside, but as far as I know, only 1 or 2 ever have and neither got in.
It is unpleasant. However, schools which get the very top results, which is what parents want from those schools, can only achieve them with top kids. If they allowed a broader range of children in, results would be lower......and many schools do just that, which is fine. Parents sending their children to very academic schools need to ensure they know the school policy and are prepared for the possibility of having to look elsewhere.....I think some parents simply refuse to hear the message which is or should be given from the outset.....and then they are cross.
The senior school of the Prep that my DC are at also has requirements for continuing to the 6th Form - I think this is pretty common. Most years 1 or 2 seem to not meet the grade. The requirements have been laid out clearly from the beginning and information is given to anyone t risk of not getting the grades at least 2 years in advance......but still parents seem shocked and outraged. I can see that if you have paid £X for many years, it feels awful, but if the information has been given clearly and in good time, I think schools can do this. In our school I know it is done gently and not lightly. Still awful to be on the receiving end though.