Regarding Common Entrance....you will find that most schools don't actually use it to make offers anymore or even as the basis of receiving the place. Schools which take in at 13 need to have made all their offers and had acceptances long before CE tests are taken and marked. They offer based on their own internal tests or using the electronic reasoning tests which are set for this purpose for independent schools and often taken in Preps. As has been said, increasingly these tests are sat in Yr6 and are effectively the same as 11+ but lead to a deferred entry offer for 13. CE is then taken in some Prep schools becaue it is a historic thing, it gives a continued focus to Yrs 7 and 8 which can be needed, especially if secondary offers have already been received and as has been said, no-one fails it. Certainly children get different levels but it is extremely rare for a child who has had an offer before CE (and all will have offers before this) to then be turned away following CE. If you look at admissions pages of schools which take in at 13 you will see that offers are made lomg before this. Most say something like 'CE results are used for internal setting purposes' - in actual fact they are often ignored totally, but still sat as a historic thing and to give Prep schools something to say they are offering in those last 2 years. 13+ senior also always have provision in admissions for those not able to do CE because the school they are in prior to this doesn't do it - showing how non-essential it is. The curriculum of CE is very content heavy and has increasingly been under scrutiny in terms of if it is useful or appropriTe as preparation for GCSE and the content and particularly skills which are needed there. Many places see it as dated and not a good preparation for the next phase. Indeed, if you look at some Preps which go to 13, some are dropping it altogether and developing their own curriculums, often in conjunction with the Seniors they feed, and with their support.
Essentially, we are in a time of transition for independent senior school start points and Prep ending points, especially for day schools. The move is very much for most children to leave Prep at 11 and for Seniors to take in the bulk of their intake then. The direction of travel is for this to continue and grow, reducing those in Preps until 13. Thing shave changed a lot in the last 5 years and will have changed again in 2 or 3 years, so bear in mind the direction of travel and that if your DC is in Yr 4 or 5 and you are thinking about this, by the time they are in Yr8, things will have moved on and that 11+ trend will have moved on too.
Yes, there will still be a small number of schools which exclusively take in at 13+. Yes, there will still need to be Preps to feed them, but many Preps which send most children to day schools simply won't be able to sustain the Yr7 and 8 sections for much longer, as numbers dwindle due to parents feeling under pressure not to miss the boat when there are more places available at 11+ and where day schools certainly will have fewer places available for 13+ and certainly for 13+ without a pre-test at 11. If a Prep is largely feeding boarding schools, then it's Yr7 and 8 has a better future ahead than if it mostly feeds day schools, or if it feeds schools which are gradually shifting their intakes towards more 11+.....and loads are doing this.
Some Seniors would really like to wholly move to 11+ intake. However they maintain Small 13+ to maintain good relations with the Preps...that really is the only reason for it in many cases, when Seniors know they can more than fill up at 11+. Preps can se the direction of travel in all of this and every year some drop their Yr7 and 8 or at least start pushing their children to do 11+ early testing for. 13+ places because they can see that leaving the decisions later and only being tested in yr8 really does reduce the options for most children and this change has been moving frighteningly fast.
I said before, and I'll say again, ask very specific questions of all the seniors you are interested in about numbers of places available at each entry point, applications per place and what they would recommend. Rely on what the seniors say most, as well as listening to advice from Preps. Look carefully at the admissions pages from the senior schools rather than just relying on the Preps. Make sure you understand the local market and what is available when and I which quantities.
And finally, one thing I think that needs to change a bit, is the idea of totally relying on the Prep school to lead parents through the choice and decision making process. Prep parents rightfully think they have paid for expert advice and insider knowledge and expect tailored advice about suitable schools. That is all correct. However, some Prep parents also seem to think this means that they have no responsibility or ability to carry out their own research. In the end, Prep Heads advise parents, but parents make the decisions for their children, and need to do so in receipt of full information. Too many I think, leave decision making too late....they trundle along with the idea that the Prep is driving it all and will inform them and make them aware of everything, and then they find they are towards the end of Yr5 and haven't really made any decisions and so facing exams in Yr6 seems too soon, so they feel relieved they have the choice to wait until Yr8, and think they lucky to have this luxury of ignoring the growing 11+ furore they hear about from parents in other schools. And then later in Yr 6 as other parents receive offers and people start talking more about where they will be going either at the end of yr6 or at the end of yr 8 based on a ore-test already sat, they then start to feel a bit uncomfortable that maybe they should have focused a hit sooner on it all and bittern the bullet and sat those earlier exams. It's not nice to spend Yr7 and 8 feeling you might have missed the boat. It doesn't mean you have to leave at 11 but I think that these days you need to have sat exams in Yr6 and got offers then, which means that you need to focus on making choices before mid Yr5, especially if you think any extra preparation might be needed, to allow time to do it, or to get fully involved in any programme the Prep is offering, or to ask for something tailor made if they aren't offering enough. 13+ Preps can give parents a sense that time is infinite in the decision making process and also that they don't need to think about it because the school will do everything,mbut in the end, parents need to decide and to do so in a timely manner. Already on this thread some parents say they have missed the 10+ boat or whatever. It's fine to decide not to get in a particular boat, but being aware of Ll the options and implications seems really important.