Private school or state school can't not access a child's medical records. They are confidential documents and can only be disclosed to other professionals with parental consent or where there are serious safeguarding concerns (involvement of Local Authority).
Private schools generally have a clause in their Ts and Cs that you must disclose reports to them and if you don't, and then they can't meet your child's needs they can withdraw your child's place. What they can't say is if you don't disclose a report they will withdraw place (because they would run the risk of indirect discrimination claims).
The fact you are posting on Mumsnet suggests to me this is not just a case that you don't think this condition is not relevant anymore, because if that was the case if it ever came out later you would simply say, "not relevant so haven't mentioned". Or you would have told the school and just said, "but it's not relevant.
I suspect you haven't told school, because of concerns your child may not have got place otherwise, and that is not necessarily untrue, because unfortunately, while no school can say "we don't accept children with x condition", many private schools ultimately will discriminate children with SEN (often without actually meeting child etc and establishing if the SEN will in fact impact on their ability to provide education).
In terms of the school, I wouldn't worry. If in the future they find out, it will likely be because something has happened so the ADHD symptoms are at a level where it does matter. The school can't withdraw your child's place, because they have ADHD. Yes they may say they wouldn't have offered, if they had seen report and noted areas of weakness, but in the same way if ultimately if your child starts suffering difficulties and they are a certain type of school they will suggest you leave in any event.
My advice would be keep a very close on eye on this situation. Intelligent neurodiverse girls learn to mask. This is often at the expense of their mental health and they can burn out in teenager years or suffer other mental health conditions. If your child is at a school where you suspect neurodiversity is not supported then the staff are less likely to be aware of subtle presentations and their actions may make things worse. Keep open to the possibility you may have to move your child to a more supportive school (and do this sooner then later if you have concerns).