Academy Assassin WAS a Class of Dark Souls 3 during the Dark Souls 3 Network Test. It did not make it into the final version of the game.
The Academy Assassin may be linked to the Vinheim Dragon School later renamed Melfian Magic Academy.
The Melfian Magic Academy, although not a playable area in Dark Souls 2, was located near Drangleic.
Given the description from the Slumbering Dragon Ring in DS1 and the Lingering Dragoncrest Ring in DS1 and DS2, respectively:
"This ring was secretly worn by a surreptitious sorcerer at Vinheim Dragon School.
The ring is engraved with an everlasting dragon in silent slumber, and masks all sound emitting from its wearer."
"A special ring granted to only the most accomplished sorcerers at Vinheim Dragon School. The ring is engraved with a lingering dragon, and boosts the length of the effects of sorceries."
"A ring used long, long ago in a land that existed where Drangleic is now. Extends the length of spell effects. The ancient dragons were once worshiped in several nations, and rumors concerning objects of similar enchantments to this uncommon ring abound in many lands, though their origins are no longer verifiable."
The item description for the Soul Greatsword from DS2, a spell used by the Academy Assassin:
"Use of this sorcery is not too dissimilar to swordplay, making it popular with the magic warriors of Melfia."
Speculation can be made that the Academy Assassin is related to the Melfian Magic Academy or any future iteration of it.