Speak with Apothecary Dithers at the Bulwark, Western Plaguelands.
On the island of Caer Darrow lies Scholomance - a school that serves as a stronghold for the Scourge. Our enemy dwells within, safely allowed to pursue research that strengthens their power. A door bars the way, and try as we might no entry is to be had without a proper key.
Apothecary Dithers is well versed on Scholomance, and has an idea how to obtain a key. You have proved your value to me as someone who can get things done; I now entrust this task to you. Speak with him on this matter.