How did that wizard reach 8th level if he never left the academy?
Levels above first are an indication that some adventuring, and thus experience at facing danger has occured.
If that's how you interpret it, there should be nothing wrong with the Commoner class. They've even faced danger, and thus gotten better at things adventurers are good at (while still focusing on their profession), or they haven't and are still level 1.
Personally, I don't see why an NPC wizard couldn't have grown very powerful without facing danger. Danger is just a catalyst for faster XP gain, it isn't a requirement. That's how it works IMC, anyway.