We are OOC, although around 10% of the cohort each year go on to GS. Like you many don't mention it, but I decided to go for a straightforward approach. In year 4 before I started any form of coaching I spoke to the teacher and asked for an honest opinion on whether DC was capable of doing well in GS .
Last summer yr 5 we said what we had been doing, in discussions with the teacher around DC's improved performance in maths which turned out to be an unexpected and welcome by product. In yr 6 start of year parents evening the teacher seemed to know that a number of DC were preparing for exams, as she mde a comment about it before we mentioned it (not that she followed this up with any suggestion that we should compensate by holding back with all of the increased school homework that came upon us at that time....) then just before we put the form in we had a discussion with the headmaster about school choice so it came up then.
We didn't expect any support from our primary school relating to coaching, and it definitely didn't make any difference to the school in knowing what we were doing, but the teacher's positive comments about ability stuck in my mind, which helped, and at least I didn't feel that parents evenings were a case of "don't mention the you-know what..." so I would advise talk to the teacher, even if you don't want to talk in the playground (a very different kettle of fish imo