I agree with all the replies here.
Whilst I am aware we are hearing your perspective only, and not the schools, or not seeing her ourselves, I genuinely can't see what the school are thinking.
As CarpetGlass suggests, one of the diagnostic criteria for ADHD is that it is present at all times, in all places.
She sits away from other children during all desk based activity
Wow! I am shocked at this. I cannot see any justification for this at all. Yes, potentially if a child with sensory issues prefers it (there are children with sensory differences, often those on the autistic spectrum who prefer to be isolated due to getting sensory overload). Or potentially as a 'one off' for a child who has ignored warnings and is being particularly distracting during a task others need to concentrate on, but I cannot see how any teacher can seriously isolate a child for half of every day
(Speaking as a teacher here). Particularly one who is so young. I would really be questioning that.
Thanks for answering my first questions - I understand transition times in the way you have described them, but thought they must be referring to someone else, as the time when moving from the mat to the table or the table to the line or tidy up time, or what ever, is the time when dc are allowed to talk , chat, move about. that is why I thought it might be a term used in a different way in this school.
I agree with everything SaltPans has written.