Sorry, just realised I didn't address part of your question.
You say when you were at school, naughty kids were just disciplined and then "behaved".
There are three types of ADHD - hyperactive, inattentive and combined. Inattentive ADHD is much harder to spot. It is just as disabling and again, see my previous comments re brain chemistry and the effect that stimulants have on us.
I wasn't diagnosed as school. I was a high achiever. Generally a "good girl". Really good grades. Behaved in class. You would never have known.
And yet. Horribly disorganised. Lost everything. Constantly late. Struggled remembering what I needed to have and when. Struggled to concentrate/focus. Struggled to know what to do in unstructured settings. Overwhelmed by anxiety. Just overwhelmed. So, so overwhelmed.
You. Wouldn't. Have. Known.
You would just have seen a quiet, pleasant girl who got on with her circle of friends, got her head down and seemed to do OK in school. You wouldn't have seen the constant turmoil that went on inside trying to navigate the most basic stuff that other kids seems to do without even thinking. You wouldn't have known how utterly USELESS I felt all of the time because basic life skills seemed so bloody impossible. I had to work so sodding hard just to get by, while constantly being criticised for messiness, being disorganised, arriving late to class, leaving homework until the very, very last minute (poor time management skills and inability to focus).
I developed really unhealthy coping mechanisms which have stayed with me for life, and have caused so many problems.
I'm a professional now and have a good career. If you met me and you knew nothing about ADHD other than the stereotype of loud, disruptive boys who misbehave, you'd never know. But privately, life is an uphill battle. I struggle so much in so many ways but only those who are very very close to me know.
But the thing is, there are lots of ADHDers just like me. You probably went to school with lots of us, and just never noticed how hard we found things. Like me, they probably weren't diagnosed but even if they were, you probably wouldn't have realised.