> She said it needed a comma in a certain sentence that would otherwise be confusing

I agree with your teacher about arguing on punctuation and the importance of a correct one.
Who studies Latin language knows how a comma may totally change the meaning of a sentence:

"Ibis, redibis, non morieris in bello"

is not the same as:

"Ibis, redibis non, morieris in bello"

I personally wrote an essay on an Italian poet who rewrote one of his four-sentences poems three times, every time by changing an element in punctuation or shifting a word to the next row.
The poem is about soldiers in WWI, comparing them to falling leaves in autumn. Every rewrite added a stronger sense of uncertainty, every time reinforcing the feeling that everything's going to fall down in a whisper.

Of course, arguing should lead to some kind of conclusion (not necessarily an agreement).
By the way, which was the sentence?

AP.

P.S.
Roughly translated, the Latin sentence is "Went, came back, did not die in war"; shifting the comma changes the position of the negation, so it becomes "Went, did not come back, died in war".
The change is minimal but the conclusion is dramatic, isn't it?