Just occured to me that this strip (http://www.giantitp.com/comics/oots0633.html) gives another reason to suppose that V's second banned school is Necromancy.

The IFCC fiends say that they specifically picked the three souls they did in order to supplement V's magical weaknesses.

We see that the third soul is a sorceror-achmage, thus bypassing the requirement to prepare in advance that is a wizard's great weakness.

The first soul is a conjuror/teleporter, which we know is one of the domains V is banned from.

Only logical to suppose that the second soul (a necromancer) was chosen because she fills V's second banned school.