Some schools write their own exams. Some use ISEB. Some use an exam designed for a consortium of schools. Some have NFER or CEM produce a bespoke test for their school. They all differ.
Each year schools will have a different ‘pass’ or ‘offer’ mark because the cohort will vary in size a bit and they always want to fill up. Some years you’d get in and others you wouldn’t.
Not everyone uses the same methods to prepare. Many don’t use ATOM and many kids have never done a CAT test. Many of the school entrance exams don’t equate to these. For example, many independent schools have a story writing exercise which doesn’t have an equivalent ATOM test.
The significant variation in the methods used by schools is really important to understand because you can be prepping for a skill that is required by one school and not others you are applying to. You could be really good at one those of skill and not at another. Consequently there isn’t a straightforward answer and especially if you’re applying to several schools which use different methods of testing, having certainty is very difficult. The idea that there is a particular test you can do in advance that will tell you if you’ll get in, is just far too simplistic.
Tutors who regularly prep for certain local schools often havea good idea when they assess a student. Things like ATOM can be used as general prep for many schools and work on general VR and NVR skills which can be useful or less so for some tests. Of course, if you’re using ATOM or have done CAT tests or other equivalent tests, the higher the score, the better the prospects.
Bear in mind, that even the very top schools have a ‘tail’ and not everyone is at the same level. Some schools have a longer tail than others and many parents would be surprised to know that even the top schools always have some students that they really don’t want returning for A Level, because they have struggled to get 8/9 at GCSE and sometimes 7 too. Not everyone is super super bright, even in the most selective schools. The tests and entrance processes are not an exact science.