I'm afraid that the evidence from a wide range of studies that setting only benefits the most able is roundly ignored by most schools. Beechen claims not to set got maths and English in year seven and to teach in mixed ability groups for these subjects, or at least it did when I looked at their prospectus when choosing schools. This is pretty misleading though. Beechen is very unusual in using CATs taken before starting at the school to stream from the outset.
The problem with streaming, where they are (correct me if I'm wrong) placed in teaching groups for most or all subjects based on test results in some subjects is that children might be able in maths but not English and find themselves not being taught to their strength because of the stream they are in. The website says "very occasionally, mistakes are made in the groupings and boys nice groups" suggesting that they have a lot of faith in those decisions and kids are pretty much stuck. This is not good at all.
I would be unsurprised if some of the low level disruption reported by Ofsted, and anecdotally by friends of mine, comes from a sense of despondency at not getting Work to challenge them or seeing a possibility of moving. Setting only works well when it's very fluid, and still shows benefits only for the able children and negative effects for the less able.
In most other secondaries in Bath, setting occurs at Christmas at the very earliest and then only for maths. It is very challenging to reach mixed ability groups for maths, and it's a compromise. Setting in other subjects starts in year 8, but not in all subjects and schools take different approaches.
Sorry to put this in such detail, but I think it's important to understand all this.