I’ll use Marvel’s “Doctor Strange” movie to illustrate the issue.
Colleges, Universities etc around the world teach “normal” skills. Anyone who can afford it and meet basic knowledge/experience pre-reqs can study at these institutions.
Intelligence agencies, business that own particularly valuable IP, etc, train select small groups in uncommon skills and knowledge to those who are capable and can be trusted to use them in the agency’s/business’ operations.
The Ancient One is extremely selective about who she admits to Kamar-Taj and teaches the extremely rare sorcery skill because of the consequences if the magic is misused/abused. Kaecilius and his zealots are an example of a wayward student teaching sorcery to others outside Kamar-Taj.
In Midgard, teachers of secret skills are like The Ancient One – they are selective, and try to restrict the knowledge to those who are capable, worthy, and can be trusted to use the knowledge in a manner which the teacher would approve of. Otherwise they’d just open an academy, publicise it, and the skill wouldn’t be a secret.
With this in mind, I am raising the question: should the skills that characters can teach to others be limited to the list of skills that Factional Offices can teach (“public skills” for want of a term), and limit the teaching of “secret skills” to the GM controlled NPCs?