Uhh,
"A school attracts danger by its very nature. Rivals seek revenge upon the instructors or headmasters for real or imagined slights. Governments feel threatened by the concentration of power. Even the curriculum can destroy the school, when practiced by careless or over-reaching students."
Even so, it just seems unrealistic that they'd not last. I mean, military schools/fighters guilds/etc would gather the same amount of threat and danger, since all things are equal.
Anywhere you're going to have a group of wizards in proximity (for the sake of sharing libraries, resources, etc), you're going to have a group who can teach. Not to mention that arcane colleges would be
The place to go for collaborative research and theory (the arcane equivalent of academia and scholarly exploration). It just seems to be a natural conclusion, not something that's 'rare' because it's avoided.
The majority of magic is combat focused, right? At least, the spells in the book(s). So, by taking up the mantle of a spellcaster, one is accepting a certain level of risk because they are learning a tool used for combat. To an extent, then, adventuring mages (or at least going into combat) isn't going to be something you can overwhelmingly frown on.
Therefore, wizards should be willing to accept the risks involved in Mage Schools.
Besides, outside of arcane schools, the only source of being taught is with mentors. And, I just find it strange that the vast majority would originate from master/student tutelage.
Unrelated question: does the article mention Warlocks? I don't have the time right this minute to comb it. I'd be curious how an academic setting treats the path of Warlocks or sorcerers.