The conclusion isn't surprising, but empirical evidence doesn't always confirm intuition so its worth studying. The sample is from Denmark where centralized national health and educational records make a large scale study like this one (N=30,982 with detailed information on grades, genomes and mental health diagnoses on each subject) possible.
In my life experience, mental health issues are the predominant reason that someone with upper middle class upbringing and intelligent parents, who often aren't low IQ themselves, struggle to achieve socio-economic success. I've seen this scenario many times and it is an important focus in estate planning now that tax consequences are no longer a dominant consideration.
Individuals with psychiatric disorders tend to modestly underperform academically relative to individuals without them (even before they are diagnosed with the condition), although the eating disorder anorexia is an exception to that rule (possibly because they are overwhelmingly female and females tend to perform better than males in school grades, on average).
ADHD and ASD present much earlier than schizophrenia and bipolar disorder, which helps explain why they have more of an impact (although the ASD impact is surprisingly small). Major depression usually (although not so universally) presents later and is episodic rather than chronic, so it is likewise unsurprising that this has a smaller impact. Some of the depressed impact of bipolar and major depression, and the elevated impact of ADHD and ASD may be due to gender as well.
Honestly, the impact of these mental health conditions on academic performance in Denmark is quite a bit smaller than I would have expected.
Individuals with psychiatric disorders perform differently in school compared to the general population. Both genetic and environmental factors contribute to such differences. Existing studies on the association of educational attainment polygenic score (EA-PGS) with school performance were based on individuals from the general population and it is unclear if the results can be generalized to individuals with psychiatric disorders.
We studied the association of EA-PGS with school performance in 30,982 individuals of whom ~60% were diagnosed with at least one major psychiatric disorder. Comparing the results between individuals with and without psychiatric disorders, we found that the associations of EA-PGS with school performance were weaker in individuals with psychiatric disorders than in those without the disorders; the largest differences were seen in attention deficit hyperactivity disorder (ADHD) and autism spectrum disorder (ASD). We further studied the associations of family socio-economic variables (parents' education and employment) with school performance and found weaker associations in individuals with psychiatric disorders than in those without the disorders.
Overall our analyses suggest that results from studies on the associations of genetic and environmental factors with school performance in the general population cannot be fully generalized to individuals with psychiatric disorders.
Veera M. Rajagopal, et al.,"Association of educational attainment polygenic score with school grades in individuals with and without psychiatric disorders." bioRvix (July 15, 2020) doi: https://doi.org/10.1101/2020.07.15.203661