oblomov - 'cos a lot of ADHD is due to differences in brain chemicals (the jury is still out afaik about social causes), and the most common drugs used are speed - but they slow down an adhd brain (whereas, obviously, they speed up other people's), so a doctor does need the training in the drugs, side effects etc.
a lot of undiagnosed adhd people suffer from depression as well, as they struggle to cope with everyday life.
however, upsetting though it is, you can't blame the school for not noticing this - loads of kids exhibit problems that adhd kids have, so the school would have no idea if a child was adhd or not. i'm assuming that to have got the dx, then you have been doing the connors test or others like it?
for us, we started thinking dd was a bit 'different' from the age of 3, (nearly 4) so did her key worker at nursery, so did her first class teacher at school, and her second, and her third. so, by the time we got to referrals, we were pretty certain. if you only had a few comments when she was younger, and nothing since, then i would assume that she has fairly mild symptoms, and has probably learnt to 'fit in' with school expectations more as she gets older?
as serendipity said, it may well be that this dx is part of an ongoing situation. i hope that your dd finds the help that she needs.