"CEO is obsessed with [thing]" isn't much evidence that the thing in question is worthwhile. Zuckerberg was utterly enthralled with the Metaverse, and we're not having this discussion in a virtual world as legless avatars.
There's two big reasons this is such a red flag:
1) Come on. Unless you are hiring highschool graduates directly, you have other means of finding out how good candidates are. If a highschool report card tells you more about a candidate than your own interview process, you need to fire everyone involved with that process.
2) Highschool performance is highly correlated with a bunch of causes that are very undesirable things to proxy-measure in your hiring process.
In the UK, where Canonical and Mark hail from, high school performance is a statistical proxy for class (wealth). In the US, it is a statistical proxy for ethnicity as well. You need to be careful with such measures, as selecting job candidates based on class or race is both unethical and commonly illegal.
Again consider that these are high school results. A person who is born to unlucky schooling opportunities can still compensate for the learning they were deprived of by working harder in college/university or their formal career after that.