What the ivory-tower grammarians meant was, a preposition must be followed by a noun, pronoun, or a phrase equivalent to a noun – but that’s still Latin, not English. The rule for English is that a prepostion must clearly refer to a noun, pronoun, or a phrase equivalent to a noun. For direct sentences the noun goes after the preposition, but questions may be formulated by replacing the unknown noun with an interrogative pronoun (who, what, where, when, or how) and moving the preposition to the end.

That’s absolutely normal English usage, and in many cases attempts to avoid it are contorted and weird. In other cases (where…at), the preposition sounds wrong because it did not belong in the first place. That is, you would answer, “The library is two blocks north”, not “The library is at…”

“Where is it?” inquired Bob.
“Where is it at?” drooled Bob.

Adding “at” at the end of the sentence adds nothing whatsoever to the question being asked. Those “Ivory Tower Academics” you sneer at were trying to simplify things and reduce unneeded verbiage from an already simple, direct sentence. Thanks for weilding your class warfare club at something so truly insignificant as this.

(FWIW, the Harvard joke made me laugh…)