[ThinkDifferent]

On top of that, Toni Morrison's is not an adjective, it is a noun in the possessive case.

Bingo. I knew there was something fishy about claiming "Toni Morrison's" was an adjective, thanks for the explanation.

However, in spite of it's outstanding adequacies

"its". (Sorry, but it is a grammar thread...)

[RightWhale]

it's

Curses be upon MS Word auto-complete. :)

[visualops]

"Toni Morrison's" is an adjective describing and identifying "genius".
There are several posts which show correct alternative structures.
Basic rule:
"An adjective modifies a noun or a pronoun by describing, identifying, or quantifying words. An adjective usually precedes the noun or the pronoun which it modifies.
The fact that in common usage grammer slides alot doesn't change what's grammatically correct. The grammer of most people is atrocious.
This teacher pressed on the issue because a student recognizing the grammatical error would have scored an incorrect answer.
The fact that most people would not notice the error is irrelevant. If you are going to have a grammar question on a PSAT test, you better make sure you are dead right.