Really? I have seen some pretty slow students at catholic/private schools. It may be different where you are, but we have 4 catholic schools, and 3 private here. They all take whoever pays the money, no test is sat at preschool to ensure only the best get in, and I have NEVER heard of a slower being expelled for being slower than the rest, and pulling down the schools marks......
The ones that I have seen haven't had to sit a test, it is just over the course of time (and in one case the day before testing) where it is recommended to the parent that their child is not fitting in at the school and perhaps they would be more comfortable elsewhere or they have found an excuse to expel them, no matter how silly the reason.

It does happen and Queensland high schools in particular have it down to an art - mainly because of the OP system we have here. OPs do not reflect students results in a clear and performanced based way, they reflect how a students results compared to their school community taking into account how their school community fitted into their own community and how that community compares to the rest of Queensland. It is a long complicated process and all students come out in the end is a number showing how they compared with all the other schools.

Parents pay private schools and expect them to be offering something more than what a public school does (as with anything you pay for - private health etc) and I am not saying this is wrong. Private schools do offer lots of things that public schools can't offer because of the extra resources and I don't think there is anything wrong with this either. Unfortunately, even in private schools, they can't show all the extras they offer by the results of these tests so they have to work the system so that these results that their school is being judged on by parents that pay money to send their kids there reflect that they are worthy of being paid for and offering a better education to students than the free school down the road.

I think too that many private schools don't have students with difficulty accessing education in the first place because their parents obviously see some value in education to put money towards it and even in the Cath Ed system where parents who struggle financially can still send their kids to those schools, the parents still go to a lot of effort to go through the process and get their kids in which means they are more likely to value it and put effort into it.

AliBaby - I think that is an excellent point.