Anecdata time: I'm currently an exchange student in China and one of my Chinese friends told me about his attempt to make friends with two North Koreans in his class. They were apparently always staying together, sleeping in the same dorm room, going to the washroom at the same time, the works. When my friend tried to talk to them, they immediately left as if they were afraid.
I have read that this is a common tactic North Korea employs when sending people abroad for business (or, in this case, study). Their families are kept as hostages, and if any of them were trying to escape, they would all be punished. So they have to constantly check on each other for fear of their own lives and those of their families.
So North Korea doesn't need to build an elite hacker school themselves, they can just send promising candidates out of the country, make sure that they do nothing but study, and ensure their return using the threat of violence.