What magical academies are there out there in Sigil? I vaguely remembered one connected to the Fated...
However I'm trying to come with ideas for one that would teach a daughter of a rich family such as Annaerlitha Bharvatlan in the dark arts and necromancy, a girl who comes from a family that sort of resembles Vampire: the Masquerade's Giovanni and Lasombra clans. Somehow I sort of see a bit of her schooling experience as being almost like a twisted version of Hogwarts with Emily Strange going there.
The Hall of Records used to be a magical academy, but hasn't been since the Fated seized it centuries ago.
Magical societies in Sigil include the Society of the Luminiferous Aether in the Lower Ward (who are very unlikely to accept commissions from The Lady's Ward).
An appropriate tutor for dark magic would by Tyrashyk of the Broken Wand in the Clerk's Ward. See Faction War, page 17. Apparently he captures people to sacrifice to some dark god of magic in the middle of the night, but hasn't yet been caught at it.
If you want a twisted version of Hogwart's in Sigil, you'll have to invent it yourself. Perhaps Tyrashyk could be a teacher there.