The Headmaster Title – Secret Title
The Headmaster / Headmistress Title is a character-bound hidden title added in the Legion expansion.
First you will have to complete an Archaeology quest chain that starts off with the quest Academic Exploration in Dalaran. Tae'thelan Bloodwatcher hands out the next quest and can be found in Azsuna, east from the Illidari Strand.
/way Azsuna 47.5 42.5
Upon completing the quest chain, you will be awarded with a Key to Nar'thalas Academy.
Using this item in the Hall of Arcane Learning will phase you to a time-warped version of the room where you can do more mini-games such as rune drawing and picture fighting. Here you will also find Elya Azuremoon who you may duel after completing all the activities (although note that you might be able to duel her without completing any activities).
Challenge Elya Azuremoon to a duel, and if you defeat her, your character (and any others who were in a party with you) will get the Headmaster/Headmistress title.