edit: Not making light of ADHD or my own struggles. Medication and self-awareness helped me improve from a B/C student into an A student. But I do find that as an adult I'm able to function without medication because instead of constant studying for tests I'm constantly learning and applying my knowledge.
However I do agree with you that we shouldn't be branding kids as "lazy" without investigation first.
In response to sibling comments, I want to QFT this. Laziness, as a diagnosis, has no explanatory power. It's a useless descriptor. It doesn't tell you how to fix it. You can self-diagnose as lazy all you like: that's, wait for it, intellectual laziness.
Laziness is a symptom, not a cause. It's like diagnosing someone with a "cough". It may be correct, but it's pointless, because it's obvious and tells you absolutely nothing about what to do.
That being said, our ailments could probably be categorized by affecting all aspects of our life equally and in full measure of time (i.e something that doesn't fluctuate across the years is probably an untreated ailment).
A sufficiently self-managing nutter is indistinguishable from a genius.
This is also a perfect description of depression.