Can a Rock Be an Antagonist?

If given the responsibility to interact with others through reticence, then yes.

In our junior high creative writing class we were taught "man vs. nature". This leads many to believe that a mountain, or a wave, or even a rock can function like an Antagonist.

They can't.

Unless someone represents the rock or speaks for the rock, it won't come across as a character, but rather a force of nature. No one interacts with the rock which, as Chris Huntley (Dramatica co-creator) puts it:

is essential for an objective character – no push and pull between forces; no act, counter, dodge, attack, etc.

Antagonists Avoid (Or Prevent) and motivate others to Reconsider their goals. They're the voice of reticence.

So, they need a voice.

They need:

directed action with a particular purpose -- to separate [them] from "stuff happens"

A rock can be an Antagonist, if the Author attributes some reactive responsibility to the inanimate object. They need to Prevent. And they need to Reconsider.