I wasn't diagnosed until half way through college. I got mostly As until sixth grade, then a gradual decline began. I graduated high school with a 2.something GPA. I think the decline was gradual enough that I guess it didn't set off any alarm bells in the guidance office.

Also I was much more the inattentive type than the hyperactive type.

During my diagnosis I was able to bring a bunch of report cards from my entire school career. That was useful. I empathize with parents who face the dilemma of having a child evaluated, without so much data to go on but with the pressure of not wanting to wait too long in case there is an issue.