Firstly, congratulations on your DC's school offers!
I had a couple of DCs at St John's College School and wrote about the school here: www.mumsnet.com/talk/local/4574285-st-faiths-prep-vs-st-johns-college-schools
To elaborate on what I wrote previously: the school has a lovely environment in the younger years (Byron House) but the academics are ropey when the kids get older (Senior House). I found it very hard to get proper feedback on how my DCs were doing as the school reports are nothing but platitudes and the teachers are reluctant to share concrete, tangible results from assessments. One of my DCs was struggling with a core subject which the teacher concerned only disclosed to me a few months before 13+ exams; when I pushed further, I was finally able to see corroborating progress test results from a year earlier but by then it was too late to do much about it.
The 13+ exam preparation is rubbish. Unless a student is aiming for the likes of Eton or Winchester or the like, the school won't set much expectation nor do much by way of planning and setting targets. Less than two months before my DC's 13+ exams, I had one teacher begin a parent-teacher meeting with the words, "I don't know what this means for the entrance test at senior school XYZ..." I left the meeting asking myself, as a prep school teacher, isn't it the teacher's job to know?
In Year 8 (or, as the school calls it, Form 6), there is an intense burst of activity in the Michaelmas term while the students cram for 13+ exams but, once most entrance exams are over by the end of January, everything just fizzles out. Some of my DC's Google classroom pages were not updated beyond May. The school made a song and dance of introducing Spanish lessons in the latter half of Year 8 (ostensibly to give the students greater choice of MFL at their next school) but this turned out to be nothing more than just some pre-canned webpages that the students scrolled through at their leisure. The school prides itself on its so-called leavers' programme for the Year 8 students which completely replaces pretty much all teaching in the summer term with activities such as paper folding and African drumming. Nothing wrong with these activities in the right quantities, but to spend the whole school day just drumming or folding paper instead of proper classroom lessons?
I appreciate your DC is still young but, if you do want to send your DC to St John's (and the school does have its redeeming features in the form of a warm and nurturing environment for younger kids), I'd seriously recommend moving them out at 11 rather than 13 if you can. In this way, your DC is more likely to get two solid years of teaching and will have more time to settle in at her/his future senior school before GCSEs hit. Given the school doesn't do much by way of preparing for entrance exams, you'd be better off preparing your DC on your own anyway, whether at 11 or 13.
Sorry again for this rather long post but I hope it's useful to you (and other forum users).