One thing that needs to be clarified: fake mental disorders such as ADHD take NORMAL behaviour of a child and classify it as a disorder.
I am not claiming there are no such thing as legitimate mental disorders. But in order for us to classify something as a mental disorder, it must be characterized by ABNORMAL behaviour. Homosexuality is a good example. A fixation on feces would be another. But even in these cases, a solution is not likely to be found in drugging the brain with a certain chemical.
But a child not paying attention is class is an issue of DICIPLINE, not a mental disorder.