If the classes are tied to insight & understanding of a caster's discipline, that might explain why the only Headmasters on the list (Isaac, Roger) are Masters of their own discipline and Adept or better in one other.
As we saw from Sizemore's narration, he gained a lot of insight into his discipline by being exposed to Wanda's Croakamancy and thinking about the Erf in a completely new way (ie As a body).
It could be that in practicing outside their discipline and gaining insight in to these foreign skills, they understand how they tie into their OWN discipline and understand it better as well.
Nothing - never tried / never succeeded
Novice - can manage to do simple / basic things in this discipline
Adept - sort of understands the how & why of what's behind a discipline
Master - understands the nuances & subtleties of discipline
??? - understands how disciplines tie together
I think that the tie-in between levels is more to do with practice, familiarity & experience, rather than anything to do with direct leveling up = new title.
... heh. Now I'd kind of like to see a high-level caster who's still just a Novice, because they only ever do ONE thing and never think about their craft. "Unbelievable amounts of juice and power in their attack... but only ONE attack they ever use."