Yes, this all sounds very like my now 22yo dd.
Potential ADHD was flagged at age 9 - we had a private Ed Psych assessment, to which school responded very well with lots of good strategies for managing behaviour and emotions. As a result of which I figured CAMHS had better things to do than see a high-functioning 9yo, so we didn't pursue formal diagnosis at that point.
It was flagged again at sixth form, by which time her behaviour and social skills were much more within the normal range, but erratic concentration became more obvious (she was doing Maths, FM, Physics and Computer Science at A-level) and we did go the diagnosis route, albeit privately. She was prescribed meds, which she took during term-time weekdays throughout sixth form (her choice), but ultimately decided the side-effects outweighed the benefits for her, and since starting uni has been medication free.
Think carefully about what you hope to gain from a diagnosis. It may unlock medication options (which are not a magic bullet) and may unlock extra time etc in exams. My dd found it helpful inasmuch as she disappeared down a bit of an internet rabbit hole in terms of finding validation of her experiences, but she was 17 by this point and had the objectivity to not make ADHD her whole identity. She did also get to sit A-levels in a smaller, quieter room, and was given additional breaks (though not extra time).
With hindsight I'm glad we didn't go the diagnosis and medication route at age 9, but we did have a very supportive primary and later secondary school in an inner-city are with lots of social problems, which meant that the behaviours she brought to the table were not considered that big a deal in the great scheme of things and the schools were very au fait with finding ways of managing and de-escalating problems. If she'd been in a small private school with lots of well-behaved little girls, I suspect it would have all been perceived very differently, and become more of a problem. We deliberately chose a very inclusive and supportive secondary school - again, one of the really strict zero-tolerance academies would not have been a success, nor would a very pushy academic private school, even if we could have afforded it.
So context is all - see how thoughtful the school can be around her behaviour and potential issues, and and how creative and supportive they are prepared to be. If they're very on it, then a diagnosis will probably not bring that much additional support at this point. But if they're shrugging their shoulders, then a diagnosis may galvanise them to put supports in place.