From WoWWiki

The Hall of Secrets is located deep within Scholomance. It is accessed from the Headmaster's Study via the northern doorway on the upper level. Lorekeeper Polkelt can be found here, presiding over a large number of zombies. This room is possibly the toughest of the six to clear, as the zombies spread a nasty disease and explode upon death.

Subzones of Scholomance
Barov Family Vault • Chamber of Summoning • The Coven • The Great Ossuary • Hall of Secrets • Hall of the Damned • Headmaster's Study • The Laboratory • The Reliquary • The Shadow Vault • The Viewing Room • Vault of the Ravenian