It is currently unclear if there a variety of titles for different multiple master-class casters (e.g. mastery of 2 vs 3 disciplines, or if "Headmaster" indicates explicitly mastery of Thinkamancy and Lookamancy).
We know that Isaac has mastered (at least) Thinkamancy and Lookamancy, but Roger Victor Clarence is also referred to as a Headmaster, and is only a Novice of Lookamancy. This would seem to imply that "Headmaster" is a title that refers to mastery of "any two" (or all 3?) Eyemancy disciplines.